Method of forming a multijunction metamorphic solar cell assembly for space applications

ABSTRACT

A multijunction solar cell assembly and its method of manufacture including interconnected first and second discrete semiconductor body subassemblies disposed adjacent and parallel to each other, each semiconductor body subassembly including first top subcell, second (and possibly third) lattice matched middle subcells; a graded interlayer adjacent to the last middle solar subcell; and a bottom solar subcell adjacent to said graded interlayer being lattice mismatched with respect to the last middle solar subcell; wherein the interconnected subassemblies form at least a four junction solar cell by a series connection being formed between the bottom solar subcell in the first semiconductor body and the bottom solar subcell in the second semiconductor body.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/980,983 filed May 16, 2018, which was a divisional of U.S. patentapplication Ser. No. 15/249,204 filed Aug. 26, 2016, now U.S. Pat. No.9,935,209, which claims the benefit of U.S. Provisional Application No.62/288,181 filed Jan. 28, 2016.

The present application is related to U.S. patent application Ser. No.15/249,185 filed Aug. 26, 2016.

The present application is also related to U.S. Provisional PatentApplication Ser. No. 62/243,239 filed Oct. 19, 2015.

This application is also related to U.S. patent application Ser. No.14/660,092 filed Mar. 17, 2015, which is a division of U.S. patentapplication Ser. No. 12/716,814 filed Mar. 3, 2010, now U.S. Pat. No.9,018,521, which was a continuation in part of U.S. patent applicationSer. No. 12/337,043 filed Dec. 17, 2008.

This application is also related to co-pending U.S. patent applicationSer. No. 13/872,663 filed Apr. 29, 2012, which was also acontinuation-in-part of application Ser. No. 12/337,043, filed Dec. 17,2008.

This application is also related to co-pending U.S. patent applicationSer. Nos. 14/828,197 and 14/828,206 filed Aug. 17, 2015.

All of the above related applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to solar cells and the fabrication ofsolar cells, and more particularly the design and specification of amultijunction solar cell using electrically coupled but spatiallyseparated semiconductor bodies based on III-V semiconductor compounds.

Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has beenpredominantly provided by silicon semiconductor technology. In the pastseveral years, however, high-volume manufacturing of III-V compoundsemiconductor multijunction solar cells for space applications hasaccelerated the development of such technology not only for use in spacebut also for terrestrial solar power applications. Compared to silicon,Ill-V compound semiconductor multijunction devices have greater energyconversion efficiencies and generally more radiation resistance,although they tend to be more complex to properly specify andmanufacture. Typical commercial III-V compound semiconductormultijunction solar cells have energy efficiencies that exceed 27% underone sun, air mass 0 (AM0) illumination, whereas even the most efficientsilicon technologies generally reach only about 18% efficiency undercomparable conditions. The higher conversion efficiency of III-Vcompound semiconductor solar cells compared to silicon solar cells is inpart based on the ability to achieve spectral splitting of the incidentradiation through the use of a plurality of photovoltaic regions withdifferent band gap energies, and accumulating the current from each ofthe regions.

In satellite and other space related applications, the size, mass andcost of a satellite power system are dependent on the power and energyconversion efficiency of the solar cells used. Putting it another way,the size of the payload and the availability of on-board services areproportional to the amount of power provided. Thus, as payloads becomemore sophisticated, and applications anticipated for five, ten, twentyor more years, the power-to-weight ratio and lifetime efficiency of asolar cell becomes increasingly more important, and there is increasinginterest not only the amount of power provided at initial deployment,but over the entire service life of the satellite system, or in terms ofa design specification, the amount of power provided at a specificfuture time, often referred to as the “end of life” (EOL).

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures or stackedsequence of solar subcells, each subcell formed with appropriatesemiconductor layers and including a p-n photoactive junction. Eachsubcell is designed to convert photons over different spectral orwavelength bands to electrical current. After the sunlight impinges onthe front of the solar cell, and photons pass through the subcells, thephotons in a wavelength band that are not absorbed and converted toelectrical energy in the region of one subcell propagate to the nextsubcell, where such photons are intended to be captured and converted toelectrical energy, assuming the downstream subcell is designed for thephoton's particular wavelength or energy band.

The individual solar cells or wafers are then disposed in horizontalarrays or panels, with the individual solar cells connected together inan electrical series and/or parallel circuit. The shape and structure ofan array, as well as the number of cells it contains, are determined inpart by the desired output voltage and current.

The efficiency of energy conversion, which converts solar energy (orphotons) to electrical energy, depends on various factors such as thedesign of solar cell structures, the choice of semiconductor materials,and the thickness of each subcell. In short, the energy conversionefficiency for each solar cell is dependent on the optimum utilizationof the available sunlight across the solar spectrum as well as the “age”of the solar cell, i.e. the length of time it has been deployed andsubject to degradation associated with the temperature and radiation inthe deployed space environment. As such, the characteristic of sunlightabsorption in semiconductor material, also known as photovoltaicproperties, is critical to determine the most efficient semiconductor toachieve the optimum energy conversion to meet customer requirements ofintended orbit and lifetime.

The energy conversion efficiency of multijunction solar cells isaffected by such factors as the number of subcells, the thickness ofeach subcell, the composition and doping of each active layer in asubcell, and the consequential band structure, electron energy levels,conduction, and absorption of each subcell, as well as its exposure toradiation in the ambient environment over time. The designation andspecification of such parameters is a non-trivial engineeringundertaking, and would vary depending upon the specific space missionand customer design requirements. Such design variables areinterdependent and interact in complex and often unpredictable ways. Inthe terminology of some, such parameters are NOT simple “resulteffective” variables to those skilled in the art confronted with complexdesign specifications and practical operational considerations.

As a result of the selection of such design variables, electricalproperties such as the short circuit current density (J_(sc)), the opencircuit voltage (V_(oc)), and the fill factor, which determine theefficiency and power output of the solar cell, are thereby affected bythe slightest change in such design variables, and as noted above, tofurther complicate the calculus, such variables and resulting propertiesalso vary over time (i.e. during the operational life of the system).

One electrical parameter of consideration taught by the presentdisclosure is the difference between the band gap and the open circuitvoltage, or (E_(g)/q−V_(oc)), of a particular active layer. Again, thevalue of such parameter may vary depending on subcell layer thicknesses,doping, the composition of adjacent layers (such as tunnel diodes), andeven the specific wafer being examined from a set of wafers processed ona single supporting platter in a reactor run. One of the importantmechanical or structural considerations in the choice of semiconductorlayers for a solar cell is the desirability of the adjacent layers ofsemiconductor materials in the solar cell, i.e. each layer ofcrystalline semiconductor material that is deposited and grown to form asolar subcell, have similar crystal lattice constants or parameters. Thepresent application is directed to solar cells with severalsubstantially lattice matched subcells, but including at least onesubcell which is lattice mismatched, and in a particular embodiment to afive junction (5J) solar cell using electrically coupled but spatiallyseparated four junction (4J) semiconductor bodies based on III-Vsemiconductor compounds.

SUMMARY OF THE DISCLOSURE Objects of the Disclosure

It is an object of the present disclosure to provide increasedphotoconversion efficiency in a multijunction solar cell for spaceapplications over the operational life of the photovoltaic power system.

It is another object of the present disclosure to provide in amultijunction solar cell in which the selection of the composition ofthe subcells and their band gaps maximizes the efficiency of the solarcell at a predetermined high temperature (in the range of 40 to 100degrees Centigrade) in deployment in space at AM0 at a predeterminedtime after the initial deployment, such time being at least one year,and in the range of one to twenty-five years.

It is another object of the present disclosure to provide a fourjunction solar cell subassembly in which the average band gap of allfour cells in the subassembly is greater than 1.44 eV, and to couple thesubassembly in electrical series with at least one additional subcell inan adjacent solar cell subassembly.

It is another object of the present disclosure to provide a latticemis-matched five junction solar cell in which the bottom subcell isintentionally designed to have a short circuit current that issubstantially greater than current through the top three subcells whenmeasured at the “beginning-of-life” or time of initial deployment.

It is another object of the present disclosure to provide afive-junction (5J) solar assembly assembled from two four-junction (4J)solar cell subassemblies so that the total current provided by the twosubassemblies matches the total current handling capability of thebottom subcell of the assembly.

It is another object of the present disclosure to match the larger shortcircuit current of the bottom subcell of the solar cell assembly withtwo or three parallel stacks of solar subcells, i.e. a configuration inwhich the value of the short circuit current of the bottom subcell is atleast twice, or at least three times, that of the solar subcells in eachparallel stack which are connected in a series with the bottom subcell.Stated another way, given the choice of the composition of the bottomsubcell, and there by the short circuit current of the bottom subcell,it is an object of the disclosure that the upper subcell stack bespecified and designed to have a short circuit which is one-half or lessthan that of the bottom subcell.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingobjects.

Features of the Invention

Briefly, and in general terms, the present disclosure describes solarcells that include a solar cell assembly of two or more solar cellsubassemblies, each of which includes a respective monolithicsemiconductor body composed of a tandem stack of solar subcells, wherethe subassemblies are interconnected electrically to one another.

As described in greater detail, the inventors of the present applicationhave discovered that interconnecting two or more spatially splitmultijunction solar cell subassemblies can be advantageous. The spatialsplit can be provided for multiple solar cell subassembliesmonolithically formed on a single substrate and remaining as amonolithic semiconductor body with distinct characteristics.Alternatively, the solar cell subassemblies can be physically separatedor fabricated individually as separate semiconductor chips that can becoupled together electrically. (Such alternative embodiments are coveredin parallel applications noted in the Reference to RelatedApplications).

One advantage of interconnecting two or more spatially splitmultijunction solar cell subassemblies is that such an arrangement canallow accumulation of the current from the upper subcells in theadjacent semiconductor bodies into the bottom subcells which have highercurrent generation ability.

One advantage of interconnecting two or more spatially splitmultijunction solar cell subassemblies is that such an arrangement canallow the bottom subcells of different subassemblies to be connected inelectrical series, this boosting the maximum operational voltage andopen circuit voltage associated with the solar cell assembly, andthereby improving efficiency.

Further, selection of relatively high band gap semiconductor materialsfor the top subcells can provide for increased photoconversionefficiency in a multijunction solar cell for outer space or otherapplications over the operational life of the photovoltaic power system.For example, increased photoconversion efficiency at a predeterminedtime (measured in terms of five, ten, fifteen or more years) afterinitial deployment of the solar cell can be achieved.

Thus, in one aspect, a monolithic solar cell subassembly includes afirst semiconductor body including an upper first solar subcell composedof (aluminum) indium gallium phosphide ((Al)InGaP); a second solarsubcell disposed adjacent to and lattice matched to said upper firstsubcell, the second solar subcell composed of (aluminum) (indium)gallium arsenide ((Al)(In)GaAs) or indium gallium arsenide phosphide(InGaAsP); and a bottom subcell lattice matched to said second subcelland composed of (indium) gallium arsenide (In)GaAs.

The aluminum (or Al) constituent element, or indium (or In), shown inparenthesis in the preceding formula means that Al or In (as the casemay be) is an optional constituent, and in the case of Al, in thisinstance may be used in an amount ranging from 0% to 40% by molefraction. In some embodiments, the amount of aluminum may be between 20%and 30%. The subcells are configured so that the current density of theupper first subcell and the second subcell have a substantially equalpredetermined first value, and the current density of the bottom subcellis at least twice that of the predetermined first value.

Briefly, and in general terms, the present disclosure provides a fivejunction solar cell comprising:

-   (a) a first semiconductor body including:    -   an upper first solar subcell composed of a semiconductor        material having a first band gap, and including a top contact on        the top surface thereof;    -   a second solar subcell adjacent to said first solar subcell and        composed of a semiconductor material having a second band gap        smaller than the first band gap and being lattice matched with        the upper first solar subcell;    -   a third solar subcell adjacent to said second solar subcell and        composed of a semiconductor material having a third band gap        smaller than the second band gap and being lattice matched with        the second solar subcell;    -   an interlayer adjacent to said third solar subcell, said        interlayer having a fourth band gap or band gaps greater than        said third band gap; and    -   a fourth solar subcell adjacent to said interlayer and composed        of a semiconductor material having a fifth band gap smaller than        the fourth band gap and being lattice mismatched with the third        solar subcell, and including a first contact on the top surface        thereof, and a second contact on the bottom surface thereof;-   (b) a second semiconductor body disposed adjacent and parallel to    the first semiconductor body and including:    -   an upper first solar subcell composed of a semiconductor        material having a first band gap, and including a top contact on        the top surface thereof;    -   a second solar subcell adjacent to said first solar subcell and        composed of a semiconductor material having a second band gap        smaller than the first band gap and being lattice matched with        the upper first solar subcell;    -   a third solar subcell adjacent to said second solar subcell and        composed of a semiconductor material having a third band gap        smaller than the second band gap and being lattice matched with        the second solar subcell and having a bottom contact;    -   an interlayer adjacent to said third solar subcell, said        interlayer having a fourth band gap greater than said third band        gap; and    -   a fourth solar subcell adjacent to said interlayer and composed        of a semiconductor material having a fifth band gap smaller than        the fourth band gap and being lattice mismatched with the third        solar subcell, and including a first contact on the top surface        thereof, and a second contact on the bottom surface thereof        connected to the terminal of a second polarity;    -   (c) wherein the top contact of the first semiconductor body is        electrically coupled with the top contact of the second        semiconductor body and to a terminal of first polarity;    -   wherein the first contact on the top surface of the fourth solar        subcell of the first semiconductor body is electrically coupled        with the bottom contact of the third solar subcell of the second        semiconductor body; and    -   the second contact on the bottom surface of the fourth solar        subcell of the first semiconductor body is electrically coupled        with the first contact on the top surface of the fourth solar        subcell of the second semiconductor body thereof so as to form a        five junction solar cell.

In some embodiments, the interlayer in each of the first and secondsemiconductor bodies is compositionally graded to substantially latticematch the upper solar subcell on one side and the adjacent lower solarsubcell on the other side, and is composed of any of the As, P, N, Sbbased III-V compound semiconductors subject to the constraints of havingthe in-plane lattice parameter less than or equal to that of the thirdsolar subcell on the first surface and greater than or equal to that ofthe lower fourth solar subcell on the other opposing surface.

In some embodiments, the interlayer in each of the first and secondsemiconductor bodies is compositionally graded to substantially latticematch the upper solar subcell on one side and the adjacent lower fourthsubcell on the other side, and is composed of any of the As, P, N, Sbbased III-V compound semiconductors subject to the constraints of havingthe in-plane lattice parameter less than or approximately equal to thatof the third solar subcell on the first surface and greater than orapproximately equal to that of the lower fourth solar subcell on theopposing surface.

In some embodiments, the interlayer in each of the first and secondsemiconductor bodies is compositionally graded to substantially latticematch the third solar subcell on one side and the lower fourth solarsubcell on the other side, and is composed of(In_(x)Ga_(1-x))Al_(1-y)As_(y), In_(x)Ga_(1-x)P, or (Al)In_(x)Ga_(1-x)Ascompound semiconductors subject to the constraints of having thein-plane lattice parameter less than or equal to that of the third solarsubcell and greater than or equal to that of the lower fourth solarsubcell.

In some embodiments, the fourth subcell has a band gap of approximately0.67 eV, the third subcell has a band gap in the range of 1.41 eV and1.31 eV, the second subcell has a band gap in the range of 1.65 to 1.8eV and the upper first subcell has a band gap in the range of 2.0 to2.20 eV.

In some embodiments, the third subcell has a band gap of approximately1.37 eV, the second subcell has a band gap of approximately 1.73 eV andthe upper first subcell has a band gap of approximately 2.10 eV.

In some embodiments, the upper first subcell is composed of indiumgallium aluminum phosphide; the second solar subcell includes an emitterlayer composed of indium gallium phosphide or aluminum gallium arsenide,and a base layer composed of aluminum gallium arsenide; the third solarsubcell is composed of indium gallium arsenide; the fourth subcell iscomposed of germanium or InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn,InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN,and the graded interlayer is composed of (Al)In_(x)Ga_(1-x)As orIn_(x)Ga_(1-x)P with 0<x<1, and (Al) denotes that aluminum is anoptional constituent.

In some embodiments, the first and second semiconductor bodies areseparate semiconductor bodies that are disposed adjacent and parallel toeach other. The semiconductor bodies being “parallel” to each other ismeant that the bodies are disposed so that the incoming lightilluminates both the upper first solar subcell of the firstsemiconductor body and the first solar subcell of the secondsemiconductor body, and that parallel light beams traverses the stack ofsubcells of each semiconductor body.

In some embodiments, the first and second semiconductor bodiesconstitute a single semiconductor body that has been etched to form twospatially separated and electrically interconnected bodies.

In some embodiments, the selection of the composition of the subcells,band gaps and short circuit current maximizes the efficiency at hightemperature (in the range of 40 to 100 degrees Centigrade) in deploymentin space at a predetermined time after the initial deployment (referredto as the beginning of life or BOL), such predetermined time beingreferred to as the end-of-life (EOL), such time being in the range ofone to twenty-five years.

In some embodiments, the first and second semiconductor bodies furthercomprises a first highly doped lateral conduction layer disposedadjacent to and beneath the second solar subcell.

In some embodiments, the first and second semiconductor bodies furthercomprises a blocking p-n diode or insulating layer disposed adjacent toand beneath the first highly doped lateral conduction layer.

In some embodiments, the first and second semiconductor bodies furthercomprises a second highly doped lateral conduction layer disposedadjacent to and beneath the blocking p-n diode or insulating layer.

In some embodiments, the short circuit density (J_(sc)) of the first,second and third middle subcells are approximately 12 mA/cm². The shortcircuit density (J_(sc)/cm²) of the subcells may have another value fordifferent implementations.

In some embodiments, the short circuit current density (J_(sc)) of thebottom subcell is approximately 34 mA/cm².

In some embodiments, the short circuit density (J_(sc)) of the bottomsubcell is at least three times that of the first, second and thirdsubcells, and at least the base of at least one of the first, second orthird solar subcells has a graded doping.

In some embodiments, the solar cell assembly further comprises a firstopening in the first semiconductor body extending from a top surface ofthe semiconductor body to the first lateral conduction layer; a secondopening in the first semiconductor body extending from the top surfaceof the first semiconductor body to the second lateral conduction layer;and a third opening in the first semiconductor body extending from asurface of the first semiconductor body to the p-type semiconductormaterial of the bottom subcell of the first semiconductor body.

In some embodiments, the solar cell assembly further comprises a firstmetallic contact pad disposed on the first lateral conduction layer ofeach of the first and second semiconductor bodies; and a second metalliccontact pad disposed on the second lateral conduction layer of the firstsemiconductor body; and an electrical interconnect connecting the firstand second contact pads.

In some embodiments, the solar cell assembly further comprises a thirdmetallic contact pad disposed on the second lateral conduction layer ofthe second semiconductor body; a fourth metallic contact pad disposed onthe p-type semiconductor material of the bottom subcell of the firstsemiconductor body; and an electrical interconnect connecting the thirdand fourth contact pads.

In another aspect, the present disclosure provides a multijunction solarcell assembly including a terminal of first polarity and a terminal ofsecond polarity comprising first and second semiconductor bodiesincluding substantially identical tandem vertical stacks of at least anupper first and a bottom second solar subcell lattice mismatched to theupper first solar subcell in which the second semiconductor body ismounted adjacent and parallel to the first semiconductor body; a bottomcontact on the bottom second subcell of the second semiconductor bodyconnected to the terminal of second polarity; a top electric contact onboth the upper first subcells of the first and second semiconductorbodies electrically connected to the top electrical contacts to theterminal of first polarity; and an electrical interconnect connectingthe bottom second subcell of the first semiconductor body in a serieselectrical circuit with the bottom second subcell of the secondsemiconductor body so that at least a multijunction solar cell is formedby the electrically interconnected semiconductor bodies. The seriesconnection of the bottom subcell of the second semiconductor bodyincrementally increases the aggregate open circuit voltage of theassembly by 0.25 volts and the operating voltage by 0.21 volts, therebyincreasing the power output of the assembly.

In another aspect, the present disclosure provides a method of forming amultijunction solar cell assembly including a terminal of first polarityand a terminal of second polarity comprising forming a semiconductorbody including a tandem vertical stack of at least an upper first and abottom second solar subcell lattice mismatched to the upper first solarsubcell; separating the semiconductor body into first and seconddiscrete semiconductor devices, each including the tandem vertical stackof at least an upper first and a bottom second solar subcells; mountingthe second semiconductor body adjacent and parallel to the firstsemiconductor body; providing a bottom contact on the bottom secondsubcell of the second semiconductor body; connecting the bottom contacton the bottom second subcell of the second semiconductor body to theterminal of second polarity; connecting the bottom second subcell of thefirst semiconductor body in a series electrical circuit with the bottomsecond subcell of the second semiconductor body so that at least amultijunction solar cell is formed by the electrically interconnectedsemiconductor bodies; and providing a top electric contact on both theupper first subcells of the first and second semiconductor bodies andelectrically connecting the top electrical contacts to the terminal offirst polarity.

In another aspect, the present disclosure provides a multijunction solarcell subassembly including an upper first solar subcell composed of asemiconductor material having a first band gap; a second solar subcelladjacent to said first solar subcell and composed of a semiconductormaterial having a second band gap smaller than the first band gap andbeing lattice matched with the upper first solar subcell; a third solarsubcell adjacent to said second solar subcell and composed of asemiconductor material having a third band gap smaller than the secondband gap and being lattice matched with the second solar subcell; agraded interlayer adjacent to said third solar subcell, said gradedinterlayer having a fourth band gap greater than said third band gap;and a fourth solar subcell adjacent to said third solar subcell andbeing lattice mismatched with the third solar subcell and composed of asemiconductor material having a fifth band gap smaller than the fourthband gap; wherein the graded interlayer is compositionally graded tolattice match the third solar subcell on one side and the lower fourthsolar subcell on the other side, and is composed of any of the As, P, N,Sb based III-V compound semiconductors subject to the constraints ofhaving the in-plane lattice parameter less than or equal to that of thethird solar subcell and greater than or equal to that of the lowerfourth solar subcell, and wherein the average band gap of all foursubcells (i.e., the sum of the four lowest direct or indirect band gapsof the materials of each subcell divided by four) is greater than 1.44eV.

The solar cell subassembly can further include a plurality of openingsin the first semiconductor body, each of the openings extending from atop surface of the first semiconductor body to a different respectivecontact layer in the first semiconductor body. Thus, for example, afirst opening in the first semiconductor body can extend from the topsurface of the semiconductor body to the first lateral conduction layer.A metallic contact pad can be disposed on the lateral conduction layer.A second opening in the first semiconductor body can extend from the topsurface of the semiconductor body to the contact back metal layer of thebottom subcell.

In another aspect, the present disclosure provides a solar cell moduleincluding a terminal of first polarity and a terminal of second polaritycomprising a first semiconductor body including a tandem vertical stackof at least a first upper, a second, third and fourth solar subcellswhich are current matched, the first upper subcell having a top contactconnected to the terminal of first polarity and a bottom fourth solarsubcell that is current mismatched from the first, second and thirdsolar subcells; a second semiconductor body disposed adjacent to thefirst semiconductor body and including a tandem vertical stack of atleast a first upper, second and third subcells, and a bottom fourthsolar subcell that is current mismatched from the first, second andthird solar subcells; wherein the top contact of the first uppersubcells of the first and second semiconductor bodies are connected; andwherein the fourth subcell of the first semiconductor body is connectedin a series electrical circuit with the fourth subcell of the secondsemiconductor body.

In another aspect, the present disclosure provides a method of forming asolar cell module including a terminal of first polarity and a terminalof second polarity comprising forming a first semiconductor bodyincluding a tandem vertical stack of at least a first upper, a second,third and fourth solar subcells which are current matched, the firstupper subcell having a top contact connected to the terminal of firstpolarity and a bottom fourth solar subcell that is current mismatchedfrom the first, second and third solar subcells; forming a secondsemiconductor body including a tandem vertical stack of at least a firstupper, second and third subcells, and a bottom fourth solar subcell thatis current mismatched from the first, second and third solar subcells;disposing the first semiconductor body adjacent to the secondsemiconductor body and connecting the top contact of the first uppersubcells of the first and second semiconductor bodies; and connectingthe fourth subcell of the first semiconductor body in a serieselectrical circuit with the fourth subcell of the second semiconductorbody.

In some embodiments, the average band gap of all four subcells (i.e.,the sum of the four band gaps of each subcell divided by four) in eachsemiconductor body is greater than 1.44 eV, and the fourth subcell iscomprised of a direct or indirect band gap material such that the lowestdirect band gap of the material is greater than 0.75 eV.

In some embodiments, the fourth subcell is comprised of a direct orindirect band gap material such that the lowest direct band gap of thematerial is less than 0.90 eV.

In some implementations, the first semiconductor body further includesone or more of the following features. For example, there may be a firsthighly doped lateral conduction layer disposed adjacent to the fourthsolar subcell. The first semiconductor body also can include a blockingp-n diode or insulating layer disposed adjacent to and above the highlydoped lateral conduction layer. The first semiconductor body may furtherinclude a second highly doped lateral conduction layer disposed adjacentto and above the blocking p-n diode or insulating layer. A metamorphiclayer can be disposed adjacent to and above the second highly dopedlateral conduction layer.

In another aspect, a solar cell assembly includes a terminal of firstpolarity and a terminal of second polarity. The solar cell assemblyincludes a first semiconductor body including a tandem vertical stack ofat least a first upper, a second, a third and a lattice mismatchedfourth solar subcell, the first upper subcell having a top contactconnected to the terminal of first polarity. The solar cell assemblyfurther includes a second semiconductor body disposed adjacent andparallel to the first semiconductor body and including a tandem verticalstack of at least a first upper, a second, third and a latticemismatched fourth bottom solar subcells, the fourth bottom subcellhaving a bottom contact connected to the terminal of second polarity.The fourth subcell of the first semiconductor body is connected in aseries electrical circuit with the fourth subcell of the secondsemiconductor body.

In some instances, the upper first subcell of the first semiconductorbody is composed of aluminium indium gallium phosphide (AlInGaP); thesecond solar subcell of the first semiconductor body is disposedadjacent to and lattice matched to said upper first subcell, and iscomposed of aluminum indium gallium arsenide (Al(In)GaAs); and the thirdsubcell is disposed adjacent to said second subcell and is composed ofindium gallium arsenide (In)GaAs.

In some cases (e.g., for an assembly having two subassemblies), theshort circuit density (J_(sc)/cm²) of each of the first and secondsubcells is approximately 12 mA/cm². In other instances (e.g., for anassembly having three subassemblies), the short circuit density(J_(sc)/cm²) of each of the first, second and third middle subcells isapproximately 10 mA/cm². The short circuit density (J_(sc)/cm²) of thebottom subcell in the foregoing cases can be approximately greater than24 mA/cm² or greater than 30 mA/cm². However, the short circuitdensities (J_(sc)/cm²) may have different values in someimplementations.

In some embodiments, the band gap of the interlayer is in the range of1.41 to 1.6 eV throughout its thickness, and may be either constant ormay be a graded interlayer and vary throughout the thickness of theinterlayer.

In some embodiments, there further comprises a distributed Braggreflector (DBR) layer adjacent to and between the third and the fourthsolar subcells and arranged so that light can enter and pass through thethird solar subcell and at least a portion of which can be reflectedback into the third solar subcell by the DBR layer.

In some embodiments, the distributed Bragg reflector layer is composedof a plurality of alternating layers of lattice matched materials withdiscontinuities in their respective indices of refraction.

In some embodiments, the difference in refractive indices betweenalternating layers is maximized in order to minimize the number ofperiods required to achieve a given reflectivity, and the thickness andrefractive index of each period determines the stop band and itslimiting wavelength.

In some embodiments, the DBR layer includes a first DBR layer composedof a plurality of p type Al_(x)Ga_(1-x)(In)As layers, and a second DBRlayer disposed over the first DBR layer and composed of a plurality of ntype or p type Al_(y)Ga_(1-y)(In)As layers, where 0<x<1, 0<y<1, and y isgreater than x, and (In) designates that indium is an optionalconstituent.

In another aspect, the present disclosure provides a five junction solarcell comprising a pair of adjacently disposed semiconductor bodies, eachbody including an upper first solar subcell composed of a semiconductormaterial having a first band gap; a second solar subcell adjacent tosaid first solar subcell and composed of a semiconductor material havinga second band gap smaller than the first band gap and being latticematched with the upper first solar subcell; a third solar subcelladjacent to said second solar subcell and composed of a semiconductormaterial having a third band gap smaller than the second band gap andbeing lattice matched with the second solar subcell; and a fourth solarsubcell adjacent to and lattice mismatched to said third solar subcelland composed of a semiconductor material having a fourth band gapsmaller than the third band gap; wherein the average band gap of allfour subcells (i.e., the sum of the four band gaps of each subcelldivided by four) is greater than 1.44 eV.

In another aspect, the present disclosure provides a four junction solarcell subassembly comprising an upper first solar subcell composed of asemiconductor material having a first band gap; a second solar subcelladjacent to said first solar subcell and composed of a semiconductormaterial having a second band gap smaller than the first band gap andbeing lattice matched with the upper first solar subcell; a third solarsubcell adjacent to said second solar subcell and composed of asemiconductor material having a third band gap smaller than the secondband gap and being lattice matched with the second solar subcell; agraded interlayer adjacent to said third solar subcell, said gradedinterlayer having a fourth band gap greater than said third band gap;and a fourth solar subcell adjacent to said third solar subcell andcomposed of a semiconductor material having a fifth band gap smallerthan the fourth band gap and being lattice mismatched with the thirdsolar subcell; wherein the graded interlayer is compositionally gradedto lattice match the third solar subcell on one side and the lowerfourth solar subcell on the other side, and is composed of any of theAs, P, N, Sb based III-V compound semiconductors subject to theconstraints of having the in-plane lattice parameter less than or equalto that of the third solar subcell and greater than or equal to that ofthe lower fourth solar subcell; wherein the fourth subcell is comprisedof a direct or indirect band gap material such that the lowest directband gap of the material is greater than 0.75 eV. In other instances,the fourth subcell may have a direct band gap of less than 0.90 eV.

In another aspect, the present disclosure provides a method ofmanufacturing a five junction solar cell comprising providing agermanium substrate; growing on the germanium substrate a sequence oflayers of semiconductor material using a MOCVD semiconductor dispositionprocess to form a solar cell comprising a plurality of subcellsincluding a metamorphic layer, growing a third subcell over themetamorphic layer having a band gap of approximately 1.30 eV to 1.41 eV,growing a second subcell over the third subcell having a band gap in therange of approximately 1.65 to 1.8 eV, and growing an upper firstsubcell over the second subcell having a band gap in the range of 2.0 to2.20 eV.

In some embodiments, there further comprises (i) a back surface field(BSF) layer disposed directly adjacent to the bottom surface of thethird subcell, and (ii) at least one distributed Bragg reflector (DBR)layer directly below the BSF layer so that light can enter and passthrough the first, second and third subcells and at least a portion ofwhich be reflected back into the third subcell by the DBR layer.

In some embodiments, the fourth (i.e., bottom) subcell of each of thesolar cell subassemblies is composed of germanium. The indirect band gapof the germanium at room temperature is about 0.66 eV, while the directband gap of germanium at room temperature is 0.8 eV. Those skilled inthe art with normally refer to the “band gap” of germanium as 0.66 eV,since it is lower than the direct band gap value of 0.8 eV. Thus, insome implementations, the fourth subcell has a direct band gap ofgreater than 0.75 eV. Reference to the fourth subcell having a directband gap of greater than 0.75 eV is expressly meant to include germaniumas a possible semiconductor material for the fourth subcell, althoughother semiconductor materials can be used as well. In other instances,the fourth subcell may have a direct band gap of less than 0.90 eV. Forexample, the fourth subcell may be composed of InGaAs, GaAsSb, InAsP,InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, or other III-V orII-VI compound semiconductor materials.

In some embodiments, additional layer(s) may be added or deleted in thecell structure without departing from the scope of the presentdisclosure.

Some implementations can include additional solar subcells in one ormore of the semiconductor bodies.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingsummaries.

Additional aspects, advantages, and novel features of the presentdisclosure will become apparent to those skilled in the art from thisdisclosure, including the following detailed description as well as bypractice of the disclosure. While the disclosure is described below withreference to preferred embodiments, it should be understood that thedisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalapplications, modifications and embodiments in other fields, which arewithin the scope of the disclosure as disclosed and claimed herein andwith respect to which the disclosure could be of utility.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better and more fully appreciated by reference tothe following detailed description when considered in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a graph representing the BOL value of the parameterE_(g)/q−V_(oc) at 28° C. plotted against the band gap of certain ternaryand quaternary materials defined along the x-axis;

FIG. 2A is a cross-sectional view of a first embodiment of a firstsemiconductor body including a four junction solar cell after severalstages of fabrication including the growth of certain semiconductorlayers on the growth substrate up to the contact layer and etchingcontact steps on lower levels according to the present disclosure;

FIG. 2B is a cross-sectional view of a first embodiment of a firstsemiconductor body of FIG. 2A after the next step of depositing a metalcontact on the top surface;

FIG. 2C is a cross-sectional view of a second embodiment of a firstsemiconductor body of FIG. 2A;

FIG. 3 is a cross-sectional view of a first embodiment of a secondsemiconductor body including a four junction solar cell after severalstages of fabrication including the growth of certain semiconductorlayers on the growth substrate up to the contact layer, according to thepresent disclosure;

FIG. 4 is a cross-sectional view of an embodiment of a five junctionsolar cell after following electrical connection of the first and secondsemiconductor bodies according to the present disclosure; and

FIG. 5 is a schematic diagram of the five junction solar cell of FIG. 4.

GLOSSARY OF TERMS

“III-V compound semiconductor” refers to a compound semiconductor formedusing at least one elements from group III of the periodic table and atleast one element from group V of the periodic table. III-V compoundsemiconductors include binary, tertiary and quaternary compounds. GroupIII includes boron (B), aluminum (Al), gallium (Ga), indium (In) andthallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb) and bismuth (Bi).

“Band gap” refers to an energy difference (e.g., in electron volts (eV))separating the top of the valence band and the bottom of the conductionband of a semiconductor material.

“Beginning of Life (BOL)” refers to the time at which a photovoltaicpower system is initially deployed in operation.

“Bottom subcell” refers to the subcell in a multijunction solar cellwhich is furthest from the primary light source for the solar cell.

“Compound semiconductor” refers to a semiconductor formed using two ormore chemical elements.

“Current density” refers to the short circuit current density J_(sc)through a solar subcell through a given planar area, or volume, ofsemiconductor material constituting the solar subcell.

“Deposited”, with respect to a layer of semiconductor material, refersto a layer of material which is epitaxially grown over anothersemiconductor layer.

“End of Life (EOL)” refers to a predetermined time or times after theBeginning of Life, during which the photovoltaic power system has beendeployed and has been operational. The EOL time or times may, forexample, be specified by the customer as part of the required technicalperformance specifications of the photovoltaic power system to allow thesolar cell designer to define the solar cell subcells and sublayercompositions of the solar cell to meet the technical performancerequirement at the specified time or times, in addition to other designobjectives. The terminology “EOL” is not meant to suggest that thephotovoltaic power system is not operational or does not produce powerafter the EOL time.

“Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

“Inverted metamorphic multijunction solar cell” or “IMM solar cell”refers to a solar cell in which the subcells are deposited or grown on asubstrate in a “reverse” sequence such that the higher band gapsubcells, which would normally be the “top” subcells facing the solarradiation in the final deployment configuration, are deposited or grownon a growth substrate prior to depositing or growing the lower band gapsubcells.

“Layer” refers to a relatively planar sheet or thickness ofsemiconductor or other material. The layer may be deposited or grown,e.g., by epitaxial or other techniques.

“Lattice mismatched” refers to two adjacently disposed materials orlayers (with thicknesses of greater than 100 nm) having in-plane latticeconstants of the materials in their fully relaxed state differing fromone another by less than 0.02% in lattice constant. (Applicant expresslyadopts this definition for the purpose of this disclosure, and notesthat this definition is considerably more stringent than that proposed,for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6%lattice constant difference).

“Metamorphic layer” or “graded interlayer” refers to a layer thatachieves a gradual transition in lattice constant generally throughoutits thickness in a semiconductor structure.

“Middle subcell” refers to a subcell in a multijunction solar cell whichis neither a Top Subcell (as defined herein) nor a Bottom Subcell (asdefined herein).

“Short circuit current (I_(sc))” refers to the amount of electricalcurrent through a solar cell or solar subcell when the voltage acrossthe solar cell is zero volts, as represented and measured, for example,in units of milliamps.

“Short circuit current density”—see “current density”.

“Solar cell” refers to an electronic device operable to convert theenergy of light directly into electricity by the photovoltaic effect.

“Solar cell assembly” refers to two or more solar cell subassembliesinterconnected electrically with one another.

“Solar cell subassembly” refers to a stacked sequence of layersincluding one or more solar subcells.

“Solar subcell” refers to a stacked sequence of layers including a p-nphotoactive junction composed of semiconductor materials. A solarsubcell is designed to convert photons over different spectral orwavelength bands to electrical current.

“Substantially current matched” refers to the short circuit currentthrough adjacent solar subcells being substantially identical (i.e.within plus or minus 1%).

“Top subcell” or “upper subcell” refers to the subcell in amultijunction solar cell which is closest to the primary light sourcefor the solar cell.

“ZTJ” refers to the product designation of a commercially availableSolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

A variety of different features of multijunction solar cells (as well asinverted metamorphic multijunction solar cells) are disclosed in therelated applications noted above. Some, many or all of such features maybe included in the structures and processes associated with thenon-inverted or “upright” solar cells of the present disclosure.However, more particularly, the present disclosure is directed to thefabrication of a multijunction lattice matched solar cell grown over ametamorphic layer which is grown on a single growth substrate whichforms a solar cell subassembly. More specifically, however, in someembodiments, the present disclosure relates to multijunction solar cellsubassemblies with direct band gaps in the range of 2.0 to 2.15 eV (orhigher) for the top subcell, and (i) 1.65 to 1.8 eV, and (ii) 1.41 eVfor the middle subcells, and 0.6 to 0.9 eV direct or indirect band gaps,for the bottom subcell, respectively, and the connection of two or moresuch subassemblies to form a solar cell assembly.

The conventional wisdom for many years has been that in a monolithicmultijunction tandem solar cell, “ . . . the desired opticaltransparency and current conductivity between the top and bottom cells .. . would be best achieved by lattice matching the top cell material tothe bottom cell material. Mismatches in the lattice constants createdefects or dislocations in the crystal lattice where recombinationcenters can occur to cause the loss of photogenerated minority carriers,thus significantly degrading the photovoltaic quality of the device.More specifically, such effects will decrease the open circuit voltage(V_(oc)), short circuit current (J_(sc)), and fill factor (FF), whichrepresents the relationship or balance between current and voltage foreffective output” (Jerry M. Olson, U.S. Pat. No. 4,667,059, “Current andLattice Matched Tandem Solar Cell”).

As progress has been made toward higher efficiency multijunction solarcells with four or more subcells, nevertheless, “it is conventionallyassumed that substantially lattice-matched designs are desirable becausethey have proven reliability and because they use less semiconductormaterial than metamorphic solar cells, which require relatively thickbuffer layers to accommodate differences in the lattice constants of thevarious materials” (Rebecca Elizabeth Jones-Albertus et al., U.S. Pat.No. 8,962,993).

Even more recently “ . . . current output in each subcell must be thesame for optimum efficiency in the series—connected configuration”(Richard R. King et al., U.S. Pat. No. 9,099,595).

The present disclosure provides an unconventional four junction design(with three grown lattice matched subcells, which are lattice mismatchedto the Ge substrate) that leads to significant performance improvementover that of traditional three junction solar cell on Ge despite thesubstantial current mismatch present between the top three junctions andthe bottom Ge junction. This performance gain is especially realized athigh temperature and after high exposure to space radiation by theproposal of incorporating high band gap semiconductors that areinherently more resistant to radiation and temperature.

As described in greater detail, the present application further notesthat interconnecting two or more spatially split multijunction solarcell subassemblies (with each subassembly incorporating Applicant'sunconventional design) can be even more advantageous. The spatial splitcan be provided for multiple solar cell subassemblies monolithicallyformed on the same substrate. Alternatively, the solar cellsubassemblies can be fabricated as separate semiconductor chips that canbe coupled together electrically.

In general terms, a solar cell assembly in accordance with one aspect ofthe present disclosure, can include a terminal of first polarity and aterminal of second polarity. The solar cell assembly includes a firstsemiconductor subassembly including a tandem vertical stack of at leasta first upper, a second, third and fourth bottom solar subcells, thefirst upper subcell having a top contact connected to the terminal offirst polarity. A second semiconductor subassembly is disposed adjacentto the first semiconductor subassembly and includes a tandem verticalstack of at least a first upper, a second, third, and fourth bottomsolar subcells, the fourth bottom subcell having a back side contactconnected to the terminal of second polarity. The fourth subcell of thefirst semiconductor subassembly is connected in a series electricalcircuit with the third subcell of the second semiconductor subassembly.Thus, a five-junction solar assembly is assembled from two four-junctionsolar cell subassemblies.

In some cases, the foregoing solar cell assembly can provide increasedphotoconversion efficiency in a multijunction solar cell for outer spaceor other applications over the operational life of the photovoltaicpower system.

Another aspect of the present disclosure is that to provide a fivejunction solar cell assembly composed of two interconnected spatiallyseparated four junction solar cell subassemblies, the average band gapof all four subcells (i.e., the sum of the four band gaps of eachsubcell divided by 4) in each solar cell subassembly being greater than1.44 eV.

Another descriptive aspect of the present disclosure is to characterizethe fourth subcell as being composed of an indirect or direct band gapmaterial such that the lowest direct band gap is greater than 0.75 eV,in some embodiments.

Another descriptive aspect of the present disclosure is to characterizethe fourth subcell as being composed of a direct band gap material suchthat the lowest direct band gap is less than 0.90 eV, in someembodiments.

In some embodiments, the fourth subcell in each solar cell subassemblyis germanium, while in other embodiments the fourth subcell is InGaAs,GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi,InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN or other III-V or II-VIcompound semiconductor material.

The indirect band gap of germanium at room temperature is about 0.66 eV,while the direct band gap of germanium at room temperature is 0.8 eV.Those skilled in the art will normally refer to the “band gap” ofgermanium as 0.66 eV, since it is lower than the direct band gap valueof 0.8 eV.

The recitation that “the fourth subcell has a direct band gap of greaterthan 0.75 eV” is therefore expressly meant to include germanium as apossible semiconductor for the fourth subcell, although othersemiconductor materials can be used as well.

More specifically, the present disclosure intends to provide arelatively simple and reproducible technique that does not employinverted processing associated with inverted metamorphic multijunctionsolar cells, and is suitable for use in a high volume productionenvironment in which various semiconductor layers are grown on a growthsubstrate in an MOCVD reactor, and subsequent processing steps aredefined and selected to minimize any physical damage to the quality ofthe deposited layers, thereby ensuring a relatively high yield ofoperable solar cells meeting specifications at the conclusion of thefabrication processes.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a depositionmethod, such as Molecular Beam Epitaxy (MBE), Organo Metallic VaporPhase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD),or other vapor deposition methods for the growth may enable the layersin the monolithic semiconductor structure forming the cell to be grownwith the required thickness, elemental composition, dopant concentrationand grading and conductivity type.

The present disclosure is directed to, in one embodiment, a growthprocess using a metal organic chemical vapor deposition (MOCVD) processin a standard, commercially available reactor suitable for high volumeproduction. More particularly, the present disclosure is directed to thematerials and fabrication steps that are particularly suitable forproducing commercially viable multijunction solar cells usingcommercially available equipment and established high-volume fabricationprocesses, as contrasted with merely academic expositions of laboratoryor experimental results.

Prior to discussing the specific embodiments of the present disclosure,a brief discussion of some of the issues associated with the design ofmultijunction solar cells, and in particular metamorphic solar cells,and the context of the composition or deposition of various specificlayers in embodiments of the product as specified and defined byApplicant is in order.

There are a multitude of properties that should be considered inspecifying and selecting the composition of, inter alia, a specificsemiconductor layer, the back metal layer, the adhesive or bondingmaterial, or the composition of the supporting material for mounting asolar cell thereon. For example, some of the properties that should beconsidered when selecting a particular layer or material are electricalproperties (e.g. conductivity), optical properties (e.g., band gap,absorbance and reflectance), structural properties (e.g., thickness,strength, flexibility, Young's modulus, etc.), chemical properties(e.g., growth rates, the “sticking coefficient” or ability of one layerto adhere to another, stability of dopants and constituent materialswith respect to adjacent layers and subsequent processes, etc.), thermalproperties (e.g., thermal stability under temperature changes,coefficient of thermal expansion), and manufacturability (e.g.,availability of materials, process complexity, process variability andtolerances, reproducibility of results over high volume, reliability andquality control issues).

In view of the trade-offs among these properties, it is not alwaysevident that the selection of a material based on one of itscharacteristic properties is always or typically “the best” or “optimum”from a commercial standpoint or for Applicant's purposes. For example,theoretical studies may suggest the use of a quaternary material with acertain band gap for a particular subcell would be the optimum choicefor that subcell layer based on fundamental semiconductor physics. As anexample, the teachings of academic papers and related proposals for thedesign of very high efficiency (over 40%) solar cells may thereforesuggest that a solar cell designer specify the use of a quaternarymaterial (e.g., InGaAsP) for the active layer of a subcell. A few suchdevices may actually be fabricated by other researchers, efficiencymeasurements made, and the results published as an example of theability of such researchers to advance the progress of science byincreasing the demonstrated efficiency of a compound semiconductormultijunction solar cell. Although such experiments and publications areof “academic” interest, from the practical perspective of the Applicantsin designing a compound semiconductor multijunction solar cell to beproduced in high volume at reasonable cost and subject to manufacturingtolerances and variability inherent in the production processes andsuited for specific applications such as the space environment where theefficiency over the entire operational life is an important goal, suchan “optimum” design from an academic perspective is not necessarily themost desirable design in practice, and the teachings of such studiesmore likely than not point in the wrong direction and lead away from theproper design direction. Stated another way, such references mayactually “teach away” from Applicant's research efforts and the ultimatesolar cell design proposed by the Applicants.

In view of the foregoing, it is further evident that the identificationof one particular constituent element (e.g. indium, or aluminum) in aparticular subcell, or the specification of the thickness, band gap,doping, or other characteristic of the incorporation of that material ina particular subcell, is interdependent on many other factors, and thusis not a single, simple “result effective variable” that one skilled inthe art can simply specify and incrementally adjust to a particularlevel and thereby increase the efficiency of a solar cell at thebeginning of life or the end of life, or over a particular time span ofoperational use. The efficiency of a solar cell is not a simple linearalgebraic equation as a function of band gap, or the amount of galliumor aluminum or other element in a particular layer. The growth of eachof the epitaxial layers of a solar cell in an MOCVD reactor is anon-equilibrium thermodynamic process with dynamically changing spatialand temporal boundary conditions that is not readily or predictablymodeled. The formulation and solution of the relevant simultaneouspartial differential equations covering such processes are not withinthe ambit of those of ordinary skill in the art in the field of solarcell design.

Another aspect of the disclosure is to match the larger short circuitcurrent of the bottom subcell of the solar cell assembly with two orthree parallel stacks of solar subcells, i.e. a configuration in whichthe value of the short circuit current of the bottom subcell is at leasttwice, or at least three times, that of the solar subcells in eachparallel stack which are connected in series with the bottom subcell.Stated another way, given the choice of the composition of the bottomsubcell, and thereby the short circuit current of the bottom subcell,the upper subcell stack is specified and designated to have a shortcircuit current which is one-third or less or is one-half or less thanthat of the bottom subcell.

Even when it is known that particular variables have an impact onelectrical, optical, chemical, thermal or other characteristics, thenature of the impact often cannot be predicted with much accuracy,particularly when the variables interact in complex ways, leading tounexpected results and unintended consequences. Thus, significant trialand error, which may include the fabrication and evaluative testing ofmany prototype devices, often over a period of time of months if notyears, is required to determine whether a proposed structure with layersof particular compositions, actually will operate as intended, in agiven environment over the operational life, let alone whether it can befabricated in a reproducible high volume manner within the manufacturingtolerances and variability inherent in the production process, andnecessary for the design of a commercially viable device.

Furthermore, as in the case here, where multiple variables interact inunpredictable ways, the proper choice of the combination of variablescan produce new and “unexpected results”, and constitute an “inventivestep” in designing and specifying a solar cell to operate in apredetermined environment (such as space), not only at the beginning oflife, but over the entire defined operational lifetime.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

One aspect of the present disclosure relates to the use and amount ofaluminum in the active layers of the upper subcells in a multijunctionsolar cell (i.e. the subcells that are closest to the primary lightsource). The effects of increasing amounts of aluminum as a constituentelement in an active layer of a subcell affects the photovoltaic deviceperformance. One measure of the “quality” or “goodness” of a solar cellsubcell or junction is the difference between the band gap of thesemiconductor material in that subcell or junction and the V_(oc), oropen circuit voltage, of that same junction. The smaller the difference,the higher the V_(oc) of the solar cell junction relative to the bandgap, and the better the performance of the device. V_(oc) is verysensitive to semiconductor material quality, so the smaller theE_(g)/q−V_(oc) of a device, the higher the quality of the material inthat device. (The charge q is introduced in the denominator fornormalization purposes, since the band gap of a semiconductor materialis measured in electron volts which is dimensioned in units of energy,and thus to compare the terms with the open circuit voltage, measured involts, the band gap is divided by the charge in coulombs, which is1.6×10⁻¹⁹ coulombs, to produce a parameter also measured in volts).There is a theoretical limit to this difference, known as theShockley-Queisser limit. That is the best voltage that a solar celljunction can produce under a given concentration of light at a giventemperature.

The experimental data obtained for single junction (Al)GaInP solar cellsindicates that increasing the Al content of the junction leads to alarger E_(g)/q−V_(oc) difference, indicating that the material qualityof the junction decreases with increasing Al content. FIG. 1 shows thiseffect. The three compositions cited in the Figure are all latticematched to GaAs, but have differing Al composition. As seen by thedifferent compositions represented, with increasing amount of aluminumrepresented by the x-axis, adding more Al to the semiconductorcomposition increases the band gap of the junction, but in so doing alsoincreases E_(g)/q−V_(oc). Hence, we draw the conclusion that adding Alto a semiconductor material degrades that material such that a solarcell device made out of that material does not perform relatively aswell as a junction with less Al.

Thus, contrary to the conventional wisdom as indicated above, thepresent application utilizes a substantial amount of aluminum, i.e.,between 10% and 40% aluminum by mole fraction in at least the topsubcell, and in some embodiments in one or more of the middle subcellsas well. In some embodiments the amount of aluminum in each of the topthree subcells is in excess of 20% by mole fraction.

Turning to the fabrication of the multijunction solar cell assembly ofthe present disclosure, and in particular a five-junction solar cellassembly, FIG. 2A is a cross-sectional view of a first embodiment of afour junction solar cell subassembly 500 after several stages offabrication including the growth of certain semiconductor layers on thegrowth substrate, and formation of grids and contacts on the contactlayer of the semiconductor body.

As shown in the illustrated example of FIG. 2A, the bottom subcell D₁includes a substrate 600 formed of p-type germanium (“Ge”) in someembodiments, which also serves as a base layer. A back metal contact pad650 formed on the bottom of base layer 600 provides electrical contact651 to the multijunction solar cell subassembly 500.

In some embodiments, the contact 651 may be in the interior of theregion 600 and not on the bottom surface of the region 600.

In some embodiments, the bottom subcell D₁ is germanium, while in otherembodiments the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs, orSiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN,InGaSbBiN or other III-V or II-VI compound semiconductor material.

The bottom subcell D₁, further includes, for example, a highly dopedn-type Ge emitter layer 601, and an n-type indium gallium arsenide(“InGaAs”) nucleation layer 602 a. The nucleation layer 602 a isdeposited over the base layer, and the emitter layer 601 is formed inthe substrate by diffusion of atoms from the nucleation layer 602 a intothe Ge substrate, thereby forming the n-type Ge layer 601.

In the first solar cell subassembly 500 of FIG. 2A, a highly dopedlateral conduction layer 602 b is deposited over layer 602 a, and ablocking p-n diode or insulating layer 602 c is deposited over the layer602 b. A second highly doped lateral conduction layer 602 d is thendeposited over layer 602 c.

In the embodiment of FIG. 2A, a first alpha layer 603, composed ofn-type (Al)GaIn(As)P, is deposited over the lateral conduction layer 602d, to a thickness of between 0.25 and 1.0 micron. Such an alpha layer isintended to prevent threading dislocations from propagating, eitheropposite to the direction of growth into the bottom subcell D₁, or inthe direction of growth into the subcell C₁, and is more particularlydescribed in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeldet al.).

A metamorphic layer (or graded interlayer) 604 is deposited over thealpha layer 603 using a surfactant. Layer 604 is preferably acompositionally step-graded series of InGaAlAs layers, preferably withmonotonically changing lattice constant, so as to achieve a gradualtransition in lattice constant in the semiconductor structure fromsubcell D₁ to subcell C₁ while minimizing threading dislocations fromoccurring. The band gap of layer 604 is either constant throughout itsthickness, in one embodiment approximately equal to 1.42 to 1.62 eV, orotherwise consistent with a value slightly greater than the band gap ofthe middle subcell C₁, or may vary within the above noted region. Oneembodiment of the graded interlayer may also be expressed as beingcomposed of (Al)In_(x)Ga_(1-x)As, with 0<x<1, and x selected such thatthe band gap of the interlayer is in the range of at approximately 1.42to 1.62 eV or other appropriate band gap.

In the surfactant assisted growth of the metamorphic layer 604, asuitable chemical element is introduced into the reactor during thegrowth of layer 604 to improve the surface characteristics of the layer.In one embodiment, such element may be a dopant or donor atom such asselenium (Se) or tellurium (Te). Small amounts of Se or Te are thereforeincorporated in the metamorphic layer 604, and remain in the finishedsolar cell. Although Se or Te are the preferred n-type dopant atoms,other non-isoelectronic surfactants may be used as well.

Surfactant assisted growth results in a much smoother or planarizedsurface. Since the surface topography affects the bulk properties of thesemiconductor material as it grows and the layer becomes thicker, theuse of the surfactants minimizes threading dislocations in the activeregions, and therefore improves overall solar cell efficiency.

As an alternative to the use of non-isoelectronic one may use anisoelectronic surfactant. The term “isoelectronic” refers to surfactantssuch as antimony (Sb) or bismuth (Bi), since such elements have the samenumber of valence electrons as the P atom of InGaP, or the As atom inInGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactantswill not typically be incorporated into the metamorphic layer 606.

In one embodiment of the present disclosure, the layer 604 is composedof a plurality of layers of (Al)InGaAs, with monotonically changinglattice constant, each layer having a band gap, approximately in therange of 1.42 to 1.62 eV. In some embodiments, the band gap is in therange of 1.45 to 1.55 eV. In some embodiments, the band gap is in therange of 1.5 to 1.52 eV.

The advantage of utilizing the embodiment of a constant bandgap materialsuch as InGaAs is that arsenide-based semiconductor material is mucheasier to process in standard commercial MOCVD reactors.

Although the preferred embodiment of the present disclosure utilizes aplurality of layers of (Al)InGaAs for the metamorphic layer 604 forreasons of manufacturability and radiation transparency, otherembodiments of the present disclosure may utilize different materialsystems to achieve a change in lattice constant from subcell C₁ tosubcell D₁. Other embodiments of the present disclosure may utilizecontinuously graded, as opposed to step graded, materials. Moregenerally, the graded interlayer may be composed of any of the As, P, N,Sb based III-V compound semiconductors subject to the constraints ofhaving the in-plane lattice parameter greater than or equal to that ofthe third solar cell and less than or equal to that of the fourth solarcell, and having a bandgap energy greater than that of the third solarcell.

A second alpha layer 605, composed of n+ type GaInP, is deposited overmetamorphic buffer layer 604, to a thickness of between 0.25 and about1.0 micron. Such second alpha layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the subcell D₁, or in the direction of growth into thesubcell C₁, and is more particularly described in U.S. PatentApplication Pub. No. 2009/0078309 A1 (Cornfeld et al.).

Heavily doped p-type aluminum gallium arsenide (“AlGaAs”) and heavilydoped n-type indium gallium arsenide (“(In)GaAs”) tunneling junctionlayers 606, 607 may be deposited over the alpha layer 605 to provide alow resistance pathway between the bottom and middle subcells D₁ and C₁.

In some embodiments, distributed Bragg reflector (DBR) layers 608 arethen grown adjacent to and between the tunnel junction 606/607 and thethird solar subcell C₁. The DBR layers 608 are arranged so that lightcan enter and pass through the third solar subcell C₁ and at least aportion of which can be reflected back into the third solar subcell C₁by the DBR layers 608. In the embodiment depicted in FIG. 2A, thedistributed Bragg reflector (DBR) layers 608 are specifically locatedbetween the third solar subcell C and tunnel junction layers 607; inother embodiments, the distributed Bragg reflector tunnel diode layers606/607 may be located between DBR layer 608 and the third subcell C₁.In another embodiment, depicted in FIG. 2C, the tunnel diode layer670/671 are located between the lateral conduction layer 602 d and thefirst alpha layer 606.

For some embodiments, distributed Bragg reflector (DBR) layers 608 canbe composed of a plurality of alternating layers 608 a through 608 z oflattice matched materials with discontinuities in their respectiveindices of refraction. For certain embodiments, the difference inrefractive indices between alternating layers is maximized in order tominimize the number of periods required to achieve a given reflectivity,and the thickness and refractive index of each period determines thestop band and its limiting wavelength.

For some embodiments, distributed Bragg reflector (DBR) layers 608 athrough 608 z includes a first DBR layer composed of a plurality of ptype Al_(x)(In)Ga_(1-x)As layers, and a second DBR layer disposed overthe first DBR layer and composed of a plurality of p typeAl_(y)(In)Ga_(1-y)As layers, where y is greater than x, with 0<x<1,0<y<1.

On top of the DBR layers 608 the subcell C₁ is grown.

In the illustrated example of FIG. 2A, the subcell C₁ includes a highlydoped p-type aluminum gallium arsenide (“Al(In)GaAs”) back surface field(“BSF”) layer 609, a p-type InGaAs base layer 610, a highly doped n-typeindium gallium phosphide (“InGaP2”) emitter layer 611 and a highly dopedn-type indium aluminum phosphide (“AlInP2”) window layer 612. The InGaAsbase layer 610 of the subcell C₁ can include, for example, approximately1.5% In. Other compositions may be used as well. The base layer 610 isformed over the BSF layer 609 after the BSF layer is deposited over theDBR layers 608 a through 608 z.

The window layer 612 is deposited on the emitter layer 611 of thesubcell C₁. The window layer 612 in the subcell C₁ also helps reduce therecombination loss and improves passivation of the cell surface of theunderlying junctions. Before depositing the layers of the subcell B,heavily doped n-type InGaP and p-type Al(In)GaAs (or other suitablecompositions) tunneling junction layers 613, 614 may be deposited overthe subcell C₁.

The middle subcell B₁ includes a highly doped p-type aluminum (indium)gallium arsenide (“Al(In)GaAs”) back surface field (“BSF”) layer 615, ap-type Al(In)GaAs base layer 616, a highly doped n-type indium galliumphosphide (“InGaP2”) or Al(In)GaAs layer 617 and a highly doped n-typeindium gallium aluminum phosphide (“AlGaAlP”) window layer 618. TheInGaP emitter layer 617 of the subcell B₁ can include, for example,approximately 50% In. Other compositions may be used as well.

Before depositing the layers of the top cell A₁, heavily doped n-typeInGaP and p-type Al(In)GaAs tunneling junction layers 619, 620 may bedeposited over the subcell B.

In the illustrated example, the top subcell A₁ includes a highly dopedp-type indium aluminum phosphide (“InAlP”) BSF layer 621, a p-typeInGaAlP base layer 622, a highly doped n-type InGaAlP emitter layer 623and a highly doped n-type InAlP2 window layer 624. The base layer 623 ofthe top subcell A₁ is deposited over the BSF layer 621 after the BSFlayer 621 is formed over the tunneling junction layers 619, 620 of thesubcell B₁. The window layer 624 is deposited over the emitter layer 623of the top subcell A₁ after the emitter layer 623 is formed over thebase layer 622.

In some embodiments, the amount of aluminum in the top subcell A, is 20%or more by mole fraction.

A cap or contact layer 625 may be deposited and patterned into separatecontact regions over the window layer 624 of the top subcell A₁. The capor contact layer 625 serves as an electrical contact from the topsubcell A₁ to metal grid 626. The doped cap or contact layer 625 can bea semiconductor layer such as, for example, a GaAs or InGaAs layer.

After the cap or contact layer 625 is deposited, the grid lines 626 areformed via evaporation and lithographically patterned and deposited overthe cap or contact layer 625.

A contact pad 627 connected to the grid 626 is formed on one edge of thesubassembly 500 to allow an electrical interconnection to be made, interalia, to an adjacent subassembly.

As with the first solar cell subassembly 500, the subcells A₂, B₂, C₂ ofthe second solar cell subassembly 700 can be configured so that theshort circuit current densities of the three subcells A₂, B₂, C₂ have asubstantially equal predetermined first value (J1<=J2<=J3), and theshort circuit current density (J4) of the bottom subcell D₂ is at leasttwice that of the predetermined first value.

Following deposition of the semiconductor layers, the semiconductor body500 is etched so that several ledges or platforms are formed onintermediate layers so that electrical contact may be made thereto, inparticular, in one embodiment, ledges 651, 661, and 662.

To this end, the solar cell subassembly can include a plurality ofopenings in the first semiconductor body, each of the openings extendingfrom a top surface of the first semiconductor body to a differentrespective contact layer in the first semiconductor body. Such“openings” may include recesses, cavities, holes, gaps, cut-outs, orsimilar structures, but for simplicity we will subsequently just use theterm “opening” throughout this disclosure. In other implementations, wecan etch through the rear of the substrate and have all the openingscome from the back side. This approach may be more efficient as it doesnot shadow the top two or top three solar subcells, but it results in asolar epitaxial structure of only a few tens of microns in thickness.

FIG. 2B is a cross-sectional view of the multijunction solar cellsubassembly 500 of FIG. 2A after additional stages of fabricationincluding the deposition of metal contact pads on the ledges depicted inFIG. 2A.

A metal contact pad 602 e is deposited on the surface of the ledge of662 which exposes a portion of the top surface of the lateral conductionlayer 602 d. This pad 602 e allows electrical contact to be made to thebottom of the stack of subcells A₁ through C₁.

A metal contact pad 602 f is deposited on the surface of the ledge of661 which exposes a portion of the top surface of the lateral conductionlayer 602 b. This pad 602 f allows electrical contact to be made to thetop of the subcell D₁.

A metal contact 651 is further provided to a ledge 650 in the p-regionof the substitute 600, or as part of the back metal layer 660 whichallows electrical contact to be made to the p-terminal of subcell D₁.

The solar cell subassemblies, 150 and 250 are presented in a highlysimplified form, but each represent the structure 500 of FIG. 2.

A second solar cell subassembly 700, which is similar to the solar cellsubassembly 500 or 600 of FIG. 2B or 2C, respectively, may be formedwith substantially the same sequence of semiconductor layers with thesame compositions and band gaps as the corresponding layers in the firstsolar cell subassembly 500 or 600. Thus, the solar cell subassembly 700also includes multiple subcells in a tandem stack. In the illustratedexample of FIG. 3, the second solar cell subassembly 700 includes anupper first subcell (Subcell A₂), a second and third solar subcells(Subcell B₂ and C₂) disposed adjacent to and lattice matched to theupper first subcell A₂, and a bottom subcell (Subcell D₂) latticemismatched to the third subcell C₂.

As with the first solar cell subassembly 500, the subcells A₂, B₂, C₂ ofthe second solar cell subassembly 700 can be configured so that theshort circuit current densities of the three subcells A₂, B₂, C₂ have asubstantially equal predetermined first value (J1=J2=J3), and the shortcircuit current density (J4) of the bottom subcell D₂ is at least twicethat of the predetermined first value.

Since the semiconductor layers 700 through 725 in subassembly 700 aresubstantially identical to layers 600 through 625 in subassembly 500, adetailed description of them will not be illustrated or provided herefor brevity.

In order to provide access to the various layers in the second solarcell subassembly 700, various ones of the layers can be exposedpartially. Thus, as shown in the example of FIG. 3, various surfaces arepartially exposed on the left side of the subassembly 700, for example,using standard photolithographic etching techniques to etch from the topsurface of the semiconductor body 700 to the particular contact layer762, 761 and 751 of interest.

A metal contact pad 703 e is deposited on the surface of the ledge of762 which exposes a portion of the top surface of the lateral conductionlayer 702 d. This pad 702 e allows electrical contact to be made to thebottom of the stack of subcells A₂ through C₂.

A metal contact pad 702 f is deposited on the surface of the ledge of761 which exposes a portion of the top surface of the lateral conductionlayer 702 b. This pad 702 f allows electrical contact to be made to thetop of the subcell E.

A metal contact 751 is further provided as part of the back metal layer750 which allows electrical contact to be made to the p-terminal ofsubcell E.

A second embodiment of the second solar cell subassembly similar to thatof FIG. 2C is another configuration (not shown) with that themetamorphic buffer layer 604 is disposed above the tunnel diode layers706, 707 and below the DBR layers 708 (not illustrated).

The foregoing multijunction solar cell subassemblies 500, 600, or 700can be fabricated, for example, in wafer-level processes and thensingulated or diced into individual semiconductor chips. The varioussemiconductor layers can be grown, one atop another, using known growthtechniques (e.g., MOCVD) as discussed above.

Each solar cell subassembly 500, 600 and 700 also can include gridlines, interconnecting bus lines, and contact pads. The geometry andnumber of the grid lines, bus lines and/or contacts may vary in otherimplementations.

As previously mentioned, two (or more) solar cell subassemblies (e.g.,500 and 700) or chips can be disposed adjacent and parallel to oneanother and connected together electrically. For example, as shown inFIG. 4, conductive (e.g., metal) interconnections 801, 802, 803, and 804can be made between different layers of the solar cell subassemblies 500and 700. Some of the interconnections are made between different layersof a single one of the solar cell subassemblies, whereas others of theinterconnections are made between the two different solar cellsubassemblies. Thus, for example, the interconnection 801 electricallyconnects together the metal contacts 133 and 233 of the first and secondsolar cell subassemblies 150 and 250 respectively. In particular,interconnection 803 connects together a contact on the lateralconduction layer 104B of the first solar cell subassembly 150 to acontact on the lateral conduction layer 204 b of the second solar cellsubassembly 250. Similarly, the interconnection 804 connects together acontact 130 on the p-region 102 of the first solar cell subassembly 150to a contact 231 on the lateral conduction layer 204 a of the secondsolar cell subassembly 250. Likewise, the interconnection 802 connectstogether a contact 132 on the lateral conduction layer 104 b of thefirst solar cell subassembly 150 to a contact 131 on the lateralconduction layer 104 a of the first solar cell subassembly 150.

In some instances, multiple electrically conductive (e.g., metal)contacts can be provided for each of the respective contacts of thesolar cell subassemblies 150, 250. This allows each of theinterconnections 801-804 to be implemented by multiple interconnectionsbetween the solar cell subassembly layers rather than just a singleinterconnection.

As noted above, the solar cell assembly includes a first electricalcontact of a first polarity and a second electrical contact of a secondpolarity. In some embodiments, the first electrical contact 807 isconnected to the metal contact 160 on the first solar cell subassembly150 by an interconnection 805, and the second electrical contact 808 isconnected to the back metal contact 217 of the second solar cellsubassembly 250.

As illustrated in FIG. 4, two or more solar cell subassemblies can beconnected electrically as described above to obtain a multijunction(e.g. a four-, five- or six-junction) solar cell assembly. In FIG. 4,the top side (n-polarity) conductivity contact 807 and bottom side(p-polarity) conductive contact 808 for the solar cell assembly areschematically depicted respectively, at the left and right-hand sides ofthe assembly.

In the example of FIG. 4, one solar cell subassembly 150 includes anupper subcell A₁, two middle subcells B₁, C₁ and a bottom subcell D₁.The other solar cell subassembly includes an upper subcell A₂, twomiddle subcells B₂, C₂ and a bottom subcell D₂. In some implementations,each solar cell subassembly 500, 700 has band gaps in the range of 2.0to 2.20 eV (or higher) for the top subcell, and (i) 1.65 to 1.8, and(ii) 1.41 eV for the middle subcell, and 0.6 to 0.9 eV, for the bottomsubcell, respectively, Further, in some embodiments, the average bandgap of all four subcells (i.e., the sum of the four band gaps of eachsubcell divided by four) in a given solar cell subassembly 500 or 700 isgreater than 1.44 eV. Other band gap ranges may be appropriate for someimplementations.

In some instances, the fourth (i.e., bottom) subcell is composed ofgermanium. The indirect band gap of the germanium at room temperature isabout 0.66 eV, while the direct band gap of germanium at roomtemperature is 0.8 eV. Those skilled in the art with normally refer tothe “band gap” of germanium as 0.66 eV, since it is lower than thedirect band gap value of 0.8 eV. Thus, in some implementations, thefourth subcell has a direct band gap of greater than 0.75 eV. Referenceto the fourth subcell having a direct band gap of greater than 0.75 eVis expressly meant to include germanium as a possible semiconductormaterial for the fourth subcell, although other semiconductor materialscan be used as well. For example, the fourth subcell may be composed ofInGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, or other III-V or II-VIcompound semiconductor materials.

FIG. 5 is a schematic diagram of the five junction solar cell assemblyof FIG. 4.

In some implementations of a five-junction solar cell assembly, such asin the example of FIG. 5, the short circuit density (J_(sc)) of theupper first subcells (A₁ and A₂) and the middle subcells (B₁, B₂, C₁,C₂) is about 12 mA/cm², and the short circuit current density (J_(sc))of the bottom subcells (D₁ and D₂) is about 24 mA/cm² or greater. Otherimplementations may have different values.

FIG. 6 is a graph of a doping profile in the emitter and base layers inone or more subcells of the multijunction solar cell of the presentinvention. The various doping profiles within the scope of the presentinvention, and the advantages of such doping profiles are moreparticularly described in U.S. patent application Ser. No. 11/956,069filed Dec. 13, 2007, herein incorporated by reference. The dopingprofiles depicted herein are merely illustrative, and other more complexprofiles may be utilized as would be apparent to those skilled in theart without departing from the scope of the present invention.

The present disclosure like that of the parallel applications notedabove provides a multijunction solar cell that follows a design rulethat one should incorporate as many high band gap subcells as possibleto achieve the goal to increase the efficiency at high temperature EOL.For example, high band gap subcells may retain a greater percentage ofcell voltage as temperature increases, thereby offering lower power lossas temperature increases. As a result, both high temperaturebeginning-of-life (HT-BOL) and HT-EOL performance of the exemplarymultijunction solar cell, according to the present disclosure, may beexpected to be greater than traditional cells.

The open circuit voltage (V_(oc)) of a compound semiconductor subcellloses approximately 2 mV per degree C. as the temperature rises, so thedesign rule taught by the present disclosure takes advantage of the factthat a higher band gap (and therefore higher voltage) subcell loses alower percentage of its V_(oc) with temperature. For example, a subcellthat produces a 1.50V at 28° C. produces 1.50-42*(0.0023)=1.403V at 70°C. which is a 6.4% voltage loss, A cell that produces 0.25V at 28° C.produces 0.25-42*(0.0018)=0.174V at 70° which is a 30.2% voltage loss.

In view of different satellite and space vehicle requirements in termsof temperature, radiation exposure, and operational life, a range ofsubcell designs using the design principles of the present disclosuremay be provided satisfying typical customer and mission requirements,and several embodiments are set forth hereunder, along with thecomputation of their efficiency at the end-of-life. The radiationexposure is experimentally measured using 1 MeV electron fluence persquare centimeter (abbreviated in the text that follows as e/cm²), sothat a comparison can be made between the current commercial devices andembodiments of solar cells discussed in the present disclosure.

As an example, a low earth orbit (LEO) satellite will typicallyexperience radiation equivalent to 5×10¹⁴ e/cm² over a five yearlifetime. A geosynchronous earth orbit (GEO) satellite will typicallyexperience radiation in the range of 5×10¹⁴ e/cm² to 1×10 e/cm² over afifteen year lifetime.

For example, the cell efficiency (%) measured at room temperature (RT)28° C. and high temperature (HT) 70° C., at beginning of life (BOL) andend of life (EOL), for a standard three junction commercial solar cell(e.g. a SolAero Technologies Corp. Model ZTJ), such as depicted in FIG.2 of U.S. patent application Ser. No. 14/828,206, is as follows:

Condition Efficiency BOL 28° C. 29.1% BOL 70° C. 26.4% EOL 70° C. 23.4%After 5E14 e/cm² radiation EOL 70° C. 22.0% After 1E15 e/cm² radiation

For the 5J solar cell assembly (comprising two interconnectedfour-junction subassemblies) described in the present disclosure, thecorresponding data is as follows:

Condition Efficiency BOL 28° C. 30.6% BOL 70° C. 27.8% EOL 70° C. 26.6%After 5E14 e/cm² radiation EOL 70° C. 26.1% After 1E15 e/cm² radiation

The new solar cell of the present disclosure has a slightly higher cellefficiency than the standard commercial solar cell (ZTJ) at BOL at 70°C. However, more importantly, the solar cell described in the presentdisclosure exhibits substantially improved cell efficiency (%) over thestandard commercial solar cell (ZTJ) at 1 MeV electron equivalentfluence of 5×10¹⁴ e/cm², and dramatically improved cell efficiency (%)over the standard commercial solar cell (ZTJ) at 1 MeV electronequivalent fluence of 1×10¹⁵ e/cm².

The wide range of electron and proton energies present in the spaceenvironment necessitates a method of describing the effects of varioustypes of radiation in terms of a radiation environment which can beproduced under laboratory conditions. The methods for estimating solarcell degradation in space are based on the techniques described by Brownet al. [Brown, W. L., J. D. Gabbe, and W. Rosenzweig, Results of theTelstar Radiation Experiments, Bell System Technical J., 42, 1505, 1963]and Tada [Tada, H. Y., J. R. Carter, Jr., B. E. Anspaugh, and R. G.Downing, Solar Cell Radiation Handbook, Third Edition, JPL Publication82-69, 1982]. In summary, the omnidirectional space radiation isconverted to a damage equivalent unidirectional fluence at a normalisedenergy and in terms of a specific radiation particle. This equivalentfluence will produce the same damage as that produced by omnidirectionalspace radiation considered when the relative damage coefficient (RDC) isproperly defined to allow the conversion. The relative damagecoefficients (RDCs) of a particular solar cell structure are measured apriori under many energy and fluence levels. When the equivalent fluenceis determined for a given space environment, the parameter degradationcan be evaluated in the laboratory by irradiating the solar cell withthe calculated fluence level of unidirectional normally incident flux.The equivalent fluence is normally expressed in terms of 1 MeV electronsor 10 MeV protons.

The software package Spenvis (www.spenvis.oma.be) is used to calculatethe specific electron and proton fluence that a solar cell is exposed toduring a specific satellite mission as defined by the duration,altitude, azimuth, etc. Spenvis employs the EQFLUX program, developed bythe Jet Propulsion Laboratory (JPL) to calculate 1 MeV and 10 MeV damageequivalent electron and proton fluences, respectively, for exposure tothe fluences predicted by the trapped radiation and solar proton modelsfor a specified mission environment duration. The conversion to damageequivalent fluences is based on the relative damage coefficientsdetermined for multijunction cells [Marvin, D. C., Assessment ofMultijunction Solar Cell Performance in Radiation Environments,Aerospace Report No. TOR-2000 (1210)-1, 2000]. A widely accepted totalmission equivalent fluence for a geosynchronous satellite mission of 15year duration is 1 MeV 1×10¹⁵ electrons/cm².

The exemplary solar cell described herein may require the use ofaluminum in the semiconductor composition of each of the top twosubcells. Aluminum incorporation is widely known in the III-V compoundsemiconductor industry to degrade BOL subcell performance due to deeplevel donor defects, higher doping compensation, shorter minoritycarrier lifetimes, and lower cell voltage and an increased BOLE_(g)/q−V_(oc) metric. However, consideration of the EOL E_(g)/q−V_(oc)metric, or more generally, total power output over a definedpredetermined operational life, in the present disclosure presents adifferent approach. In short, increased BOL E_(g)/q−V_(oc) may be themost problematic shortcoming of aluminum containing subcells; the otherlimitations can be mitigated by modifying the doping schedule orthinning base thicknesses.

For completeness and clarification in the recitation of various claimedelements with corresponding elements depicted in the Figure, anannotated set of claims with reference numbers is set forth in thefollowing clauses:

Clause 1 . . . A multijunction solar cell assembly having a terminal ofa first polarity and a terminal of a second polarity (808) comprising:

-   (a) a first semiconductor body (500) including:    -   an upper first solar subcell (A1) composed of a semiconductor        material having a first band gap, and including a top contact        (627) on the top surface thereof;    -   a second solar subcell (B1) adjacent to said first solar subcell        (A1) and composed of a semiconductor material having a second        band gap smaller than the first band gap and being lattice        matched with the upper first solar subcell (A1);    -   a third solar subcell (C1) adjacent to said second solar subcell        (B1) and composed of a semiconductor material having a third        band gap smaller than the second band gap and being lattice        matched with the second solar subcell (B1);    -   an interlayer adjacent to said third solar subcell (C1), said        interlayer having a fourth band gap or band gaps greater than        said third band gap; and a fourth solar subcell (D1) adjacent to        said interlayer and composed of a semiconductor material having        a fifth band gap smaller than the fourth band gap and being        lattice mismatched with the third solar subcell (C1), and        including a first contact (602 f) on the top surface thereof,        and a second contact (650) on the bottom surface thereof,-   (b) a second semiconductor body (700) disposed adjacent and parallel    to the first semiconductor body (500) and including:    -   an upper first solar subcell (A2) composed of a semiconductor        material having a first band gap, and including a top contact        (727) on the top surface thereof,    -   a second solar subcell (B2) adjacent to said first solar subcell        (A2) and composed of a semiconductor material having a second        band gap smaller than the first band gap and being lattice        matched with the upper first solar subcell (A2);    -   a third solar subcell (C2) adjacent to said second solar subcell        (B2) and composed of a semiconductor material having a third        band gap smaller than the second band gap and being lattice        matched with the second solar subcell (B2) and having a bottom        contact (702 e);    -   an interlayer adjacent to said third solar subcell (C2), said        interlayer having a fourth band gap greater than said third band        gap; and    -   a fourth solar subcell (D2) adjacent to said interlayer and        composed of a semiconductor material having a fifth band gap        smaller than the fourth band gap and being lattice mismatched        with the third solar subcell (C2), and including a first contact        (702 f) on the top surface thereof, and a second contact (750)        on the bottom surface thereof connected to the terminal of a        second polarity (808);-   (c) wherein the top contact (625) of the first semiconductor body    (500) is electrically coupled with the top contact (725) of the    second semiconductor body (700) and to a terminal of first polarity    (807);    -   wherein the first contact (602 f) on the top surface of the        fourth solar subcell (D1) of the first semiconductor body (500)        is electrically coupled with the bottom contact (702 e) of the        third solar subcell (C2) of the second semiconductor body (700);    -   the second contact (650) on the bottom surface of the fourth        solar subcell (D1) of the first semiconductor body (500) is        electrically coupled with the first contact (702 f) on the top        surface of the fourth solar subcell (D2) of the second        semiconductor body (700) thereof so as to form a five junction        solar cell;    -   and wherein the interlayer in each of the first and second        semiconductor bodies (500, 700) is compositionally graded to        substantially lattice match the third solar subcell (C1, C2) on        one side and the lower fourth solar subcell (D1, D2) on the        other side, and is composed of any of the As, P, N, Sb based        III-V compound semiconductors subject to the constraints of        having the in-plane lattice parameter less than or equal to that        of the third solar subcell (C1, C2) and greater than or equal to        that of the lower fourth solar subcell (D1, D2).

Clause 2 . . . A multijunction solar cell as defined in clause 1,wherein the short circuit density of each fourth subcell (D1, D2) beingat least twice that of the solar subcells in each semiconductor bodywhich are connected in a series with the fourth subcell (D1, D2).

Clause 3 . . . A multijunction solar cell as defined in clause 1,wherein the fourth subcell (D1, D2) has a band gap of approximately 0.67eV, the third subcell (C1, C2) has a band gap in the range ofapproximately 1.41 eV and 1.31 eV, the second subcell (B1, B2) has aband gap in the range of approximately 1.65 to 1.8 eV and the upperfirst subcell (A1, A2) has a band gap in the range of 2.0 to 2.20 eV.

Clause 4 . . . A multijunction solar cell as defined in clause 1,wherein the upper first subcell (A1, A2) is composed of indium galliumaluminum phosphide;

-   -   the second solar subcell (B1, B2) includes an emitter layer        composed of indium gallium phosphide or aluminum gallium        arsenide or indium aluminum gallium arsenide, and a base layer        composed of aluminum gallium arsenide, indium gallium arsenide        phosphide or indium aluminum gallium arsenide;    -   the third solar subcell (C1, C2) is composed of indium gallium        arsenide;    -   the fourth subcell (D1, D2) is composed of germanium.

Clause 5 . . . A multijunction solar cell as defined in clause 1,wherein the interlayer is composed of p type (A1)In_(x)Ga_(1-x)As orIn_(x)Ga_(1-x)P with 0<x<1, and (A1) designates that aluminum is anoptional constituent.

Clause 6 . . . A multijunction solar cell as defined in clause 1,wherein the first (500) and second (700) semiconductor bodies furthercomprise a first highly doped lateral conduction layer (602 b) disposedadjacent to and above the fourth solar subcell (D1) and a blocking p-ndiode or insulating layer (602 c) disposed adjacent to and above thefirst highly doped lateral conduction layer (602 b), and a second highlydoped lateral conduction layer disposed (602 d) adjacent to and abovethe blocking p-n diode or insulating layer (602 c).

Clause 7 . . . A multijunction solar cell assembly as defined in clause3, wherein the third subcell (C1, C2) has a band gap of approximately1.37 eV, the second subcell (B1, B2) has a band gap of approximately1.73 eV and the upper first subcell (A1, A2) has a band gap ofapproximately 2.10 eV.

Clause 8 . . . A multijunction solar cell assembly as defined in clause1, wherein the band gap of the interlayer is in the range of 1.41 eV to1.6 eV throughout its thickness.

Clause 9 . . . A multijunction solar cell assembly as defined in clause1, further comprising:

-   -   a distributed Bragg reflector layer (608) adjacent to and        between the third (C1, C2) and the fourth (D1, D2) solar        subcells and arranged so that light can enter and pass through        the third solar subcell (C1, C2) and at least a portion of which        can be reflected back into the third solar subcell (C1, C2) by        the distributed Bragg reflector layer (608), and the distributed        Bragg reflector layer (608) is composed of a plurality of        alternating layers of lattice matched materials with        discontinuities in their respective indices of refraction and        the difference in refractive indices between alternating layers        is maximized in order to minimize the number of periods required        to achieve a given reflectivity, and the thickness and        refractive index of each period determines the stop band and its        limiting wavelength, and wherein the distributed Bragg reflector        layer (608) includes a first distributed Bragg reflector layer        (608) composed of a plurality of p type Al_(x)Ga_(1-x)(In)As        layers, and a second distributed Bragg reflector layer (608)        disposed over the first distributed Bragg reflector layer (608)        and composed of a plurality of n type or p type        Al_(y)Ga_(1-y)(In)As layers, where 0<x<1, 0<y<1, and y is        greater than x, and (In) designates that indium is an optional        constituent.

Clause 10 . . . A multijunction solar cell assembly as defined in clause2, wherein the short circuit current density (J_(sc)) of the first (A1,A2), second (B1, B2) and third middle (C1, C2) subcells areapproximately 11 mA/cm², and the short circuit current density (J_(sc))of the bottom subcell (D1, D2) is approximately 34 mA/cm².

Clause 11 . . . A multijunction solar cell assembly as defined in clause1, wherein at least the base of at least one of the first (A1, A2),second (B1, B2) or third (C1, C2) solar subcells has a graded doping.

Clause 12 . . . A multijunction solar cell assembly as defined in clause6 comprising:

-   -   a first opening in the first semiconductor body (500) extending        from a top surface of the semiconductor body to the first        lateral conduction layer (602 b);    -   a second opening in the first semiconductor body (500) extending        from the top surface of the semiconductor body (500) to the        second lateral conduction layer (602 d); and    -   a third opening in the first semiconductor body (500) extending        from a surface of the first semiconductor body (500) to the        p-type semiconductor material of the bottom subcell (D1), a        first metallic contact pad (602 f, 702 f) disposed on the first        lateral conduction layer (602 b) of each of the first (500) and        second (700) semiconductor bodies;    -   a second metallic contact pad (602 e) disposed on the second        lateral conduction layer (602 d) of the first semiconductor body        (500); and    -   an electrical interconnect connecting the first and second        contact pads, a third metallic contact pad (702 e) disposed on        the second lateral conduction layer (702 d) of the second        semiconductor body (700);    -   a fourth metallic contact pad (651) disposed on the p-type        semiconductor material of the bottom subcell (D1) of the first        semiconductor body (500); and    -   an electrical interconnect (804) connecting the third and fourth        (651) contact pads.

Clause 13 . . . A multijunction solar cell assembly having a terminal ofa first polarity (807) and a terminal of a second polarity (808)comprising:

-   (a) a first semiconductor body (500) including:    -   an upper first solar subcell (A1) composed of a semiconductor        material having a first band gap, and including a top contact        (627) on the top surface thereof;    -   a second solar subcell (B1) adjacent to said first solar subcell        (A1) and composed of a semiconductor material having a second        band gap smaller than the first band gap and being lattice        matched with the upper first solar subcell (A1);    -   a third solar subcell (C1) adjacent to said second solar subcell        (B1) and composed of a semiconductor material having a third        band gap smaller than the second band gap and being lattice        matched with the second solar subcell (B1);    -   a fourth solar subcell (D1) adjacent to said interlayer and        composed of a semiconductor material having a fifth band gap        smaller than the fourth band gap, and including a first contact        (602 f) on the top surface thereof, and a second contact (650)        on the bottom surface thereof;-   (b) a second semiconductor body (700) disposed adjacent and parallel    to the first semiconductor body (500) and including:    -   an upper first solar subcell (A2) composed of a semiconductor        material having a first band gap, and including a top contact        (727) on the top surface thereof;    -   a second solar subcell (B2) adjacent to said first solar subcell        (A2) and composed of a semiconductor material having a second        band gap smaller than the first band gap and being lattice        matched with the upper first solar subcell (A2);    -   a third solar subcell (C2) adjacent to said second solar subcell        (B2) and composed of a semiconductor material having a third        band gap smaller than the second band gap and being lattice        matched with the second solar subcell (B2) and having a bottom        contact (702 e);    -   a fourth solar subcell (D2) adjacent to said interlayer and        composed of a semiconductor material having a fifth band gap        smaller than the fourth band gap, and including a first contact        (702 e) on the top surface thereof, and a second contact (750)        on the bottom surface thereof connected to the terminal of a        second polarity (808);-   (c) wherein the top contact (625) of the first semiconductor body    (500) is electrically coupled with the top contact (725) of the    second semiconductor body (700) and to a terminal of first polarity    (807);    -   wherein the first contact (602 f) on the top surface of the        fourth solar subcell (D1) of the first semiconductor body (500)        is electrically coupled with the bottom contact (702 e) of the        third solar subcell (C2) of the second semiconductor body (700);    -   the second contact (650) on the bottom surface of the fourth        solar subcell (D1) of the first semiconductor body (500) is        electrically coupled with the first contact (702 f) on the top        surface of the fourth solar subcell (D2) of the second        semiconductor body (700) thereof so as to form a five junction        solar cell;    -   wherein the first and second semiconductor bodies further        comprise a first highly doped lateral conduction layer disposed        adjacent to and above the fourth solar subcell (D1) and a        blocking p-n diode or insulating layer disposed adjacent to and        above the first highly doped lateral conduction layer, and a        second highly doped lateral conduction layer disposed adjacent        to and above the blocking p-n diode or insulating layer.

Clause 14 . . . A multijunction solar cell as defined in clause 1,wherein the fourth subcell (D1, D2) has a band gap of approximately 0.67eV, the third subcell (C1, C2) has a band gap in the range ofapproximately 1.41 eV and 1.31 eV, the second subcell (B1, B2) has aband gap in the range of approximately 1.65 to 1.8 eV and the upperfirst subcell (A1, A2) has a band gap in the range of 2.0 to 2.20 eV.

Clause 15 . . . A multijunction solar cell as defined in clause 13,wherein the upper first subcell (A1, A2) is composed of indium galliumaluminum phosphide;

-   -   the second solar subcell (B1, B2) includes an emitter layer        composed of indium gallium phosphide or aluminum gallium        arsenide or indium aluminum gallium arsenide, and a base layer        composed of aluminum gallium arsenide, indium gallium arsenide        phosphide or indium aluminum gallium arsenide;    -   the third solar subcell (C1, C2) is composed of indium gallium        arsenide; and the fourth subcell (D1, D2) is composed of        germanium.

Clause 16 . . . A multijunction solar cell assembly as defined in clause13, further comprising:

-   -   a distributed Bragg reflector layer adjacent to and between the        third (C1, C2) and the fourth (D1, D2) solar subcells are        arranged so that light can enter and pass through the third        solar subcell (C1, C2) and at least a portion of which can be        reflected back into the third solar subcell (C1, C2) by the        distributed Bragg reflector layer (608), and the distributed        Bragg reflector layer (608) is composed of a plurality of        alternating layers of lattice matched materials with        discontinuities in their respective indices of refraction and        the difference in refractive indices between alternating layers        is maximized in order to minimize the number of periods required        to achieve a given reflectivity, and the thickness and        refractive index of each period determines the stop band and its        limiting wavelength, and wherein the distributed Bragg reflector        layer (608) includes a first distributed Bragg reflector layer        (608) disposed over the first distributed Bragg reflector layer        (608) and composed of a plurality of n type or p type        Al_(y)Ga_(1-y)(In)As layers, where 0<x<1, 0<y<1, and y is        greater than x, and (In) designates that indium is an optional        constituent.

Clause 17 . . . A multijunction solar cell assembly as defined in clause13, wherein the short circuit current density (J_(sc)) of the first (A1,A2), second (B1, B2) and third middle (C1, C2) subcells areapproximately 11 mA/cm², and the short circuit current density (J_(sc))of the bottom subcell (D1, D2) is approximately 34 mA/cm².

Clause 18 . . . A multijunction solar cell assembly as defined in clause13, wherein at least the base of at least one of the first (A1, A2),second (B1, B2) or third (C1, C2) solar subcells has a graded doping.

Clause 19 . . . A multijunction solar cell assembly as defined in clause13, comprising: a first opening in the first semiconductor body (500)extending from a top surface of the semiconductor body (500) to thefirst lateral conduction layer (602 b);

-   -   a second opening in the first semiconductor body (500) extending        from the top surface of the semiconductor body (500) to the        second lateral conduction layer (602 d);    -   a third opening in the first semiconductor body (500) extending        from a surface of the first semiconductor body (500) to the        p-type semiconductor material of the bottom subcell (D1), a        first metallic contact pad (602 f, 702 f) disposed on the first        lateral conduction layer (602 b) of each of the first (500) and        second (700) semiconductor bodies;    -   a second metallic contact pad (602 e) disposed on the second        lateral conduction layer (602 d) of the first semiconductor body        (500); and    -   an electrical interconnect connecting the first and second        contact pads, a third metallic contact pad (702 e) disposed on        the second lateral conduction layer (702 d) of the second        semiconductor body (700);    -   a fourth metallic contact pad (651) disposed on the p-type        semiconductor material of the bottom subcell (D1) of the first        semiconductor body (500); and    -   an electrical interconnect (804) connecting the third and fourth        (651) contact pads.

Clause 20 . . . A multijunction solar cell assembly including a terminalof first polarity and a terminal of second polarity comprising:

-   -   first and second semiconductor bodies including substantially        identical tandem vertical stacks of at least an upper first and        a bottom second solar subcell lattice mismatched to the upper        first solar subcell in which the second semiconductor body is        mounted adjacent and parallel to the first semiconductor body;    -   a bottom contact on the bottom second subcell of the second        semiconductor body connected to the terminal of second polarity;    -   a top electric contact on both the upper first subcells of the        first and second semiconductor bodies electrically connected to        the top electrical contacts to the terminal of first polarity;        and    -   an electrical interconnect connecting the bottom second subcell        of the first semiconductor body in a series electrical circuit        with the bottom second subcell of the second semiconductor body        so that at least a three junction solar cell is formed by the        electrically interconnected semiconductor bodies;    -   wherein the first and second semiconductor bodies further        comprise a first highly doped lateral conduction layer disposed        adjacent to and above the bottom second solar subcell and a        blocking p-n diode or insulating layer disposed adjacent to and        above the first highly doped lateral conduction layer, and a        second highly doped lateral conduction layer disposed adjacent        to and above the blocking p-n diode or insulating layer.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofstructures or constructions differing from the types of structures orconstructions described above.

Although described embodiments of the present disclosure utilizes avertical tandem stack of four subcells, various aspects and features ofthe present disclosure can apply to tandem stacks with fewer or greaternumber of subcells, i.e. two junction cells, three junction cells, fivejunction cells, etc.

In addition, although the disclosed embodiments are configured with topand bottom electrical contacts, the subcells may alternatively becontacted by means of metal contacts to laterally conductivesemiconductor layers between the subcells. Such arrangements may be usedto form 3-terminal, 4-terminal, and in general, n-terminal devices. Thesubcells can be interconnected in circuits using these additionalterminals such that most of the available photogenerated current densityin each subcell can be used effectively, leading to high efficiency forthe multijunction cell, notwithstanding that the photogenerated currentdensities are typically different in the various subcells.

As noted above, the solar cell described in the present disclosure mayutilize an arrangement of one or more, or all, homojunction cells orsubcells, i.e., a cell or subcell in which the p-n junction is formedbetween a p-type semiconductor and an n-type semiconductor both of whichhave the same chemical composition and the same band gap, differing onlyin the dopant species and types, and one or more heterojunction cells orsubcells. Subcell A, with p-type and n+ type InGaAlP is one example of ahomojunction subcell.

In some cells, a thin so-called “intrinsic layer” may be placed betweenthe emitter layer and base layer, with the same or different compositionfrom either the emitter or the base layer. The intrinsic layer mayfunction to suppress minority-carrier recombination in the space-chargeregion. Similarly, either the base layer or the emitter layer may alsobe intrinsic or not-intentionally-doped (“NID”) over part or all of itsthickness.

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and may include AlInP, AlAs, AlP, GaP, InP, GaSb, AlSb,InAs, InSb, ZnSe, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs,GaInPAs, (In)AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb,GaInSb, AlGaInSb, AlN, GaN, InN, GaInN, AlGaInN, GaInNAs, GaInNAsSb,GaNAsSb, GaNAsInBi, GaNAsSbBi, GaNAsInBiSb, AlGaInNAs, ZnSSe, CdSSe,SiGe, SiGeSn, and similar materials, and still fall within the spirit ofthe present invention.

While the solar cell described in the present disclosure has beenillustrated and described as embodied in a conventional multijunctionsolar cell, it is not intended to be limited to the details shown, sinceit is also applicable to inverted metamorphic solar cells, and variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

Thus, while the description of the semiconductor device described in thepresent disclosure has focused primarily on solar cells or photovoltaicdevices, persons skilled in the art know that other optoelectronicdevices, such as thermophotovoltaic (TPV) cells, photodetectors andlight-emitting diodes (LEDS), are very similar in structure, physics,and materials to photovoltaic devices with some minor variations indoping and the minority carrier lifetime. For example, photodetectorscan be the same materials and structures as the photovoltaic devicesdescribed above, but perhaps more lightly-doped for sensitivity ratherthan power production. On the other hand LEDs can also be made withsimilar structures and materials, but perhaps more heavily-doped toshorten recombination time, thus radiative lifetime to produce lightinstead of power. Therefore, this invention also applies tophotodetectors and LEDs with structures, compositions of matter,articles of manufacture, and improvements as described above forphotovoltaic cells.

Without further analysis, from the foregoing others can, by applyingcurrent knowledge, readily adapt the present invention for variousapplications. Such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

The invention claimed is:
 1. A multijunction solar cell assembly havinga terminal of a first polarity and a terminal of a second polaritycomprising: (a) a first semiconductor body including: an upper firstsolar subcell (A1) composed of a semiconductor material having a firstband gap, and including a top contact on the top surface thereof, asecond solar subcell (B1) adjacent to said first solar subcell (A1) andcomposed of a semiconductor material having a second band gap smallerthan the first band gap and being lattice matched with the upper firstsolar subcell (A1); a third solar subcell (C1) adjacent to said secondsolar subcell (B1) and composed of a semiconductor material having athird band gap smaller than the second band gap and being latticematched with the second solar subcell (B1); an interlayer adjacent tosaid third solar subcell (C1), said interlayer having a fourth band gapor band gaps greater than said third band gap; and a fourth solarsubcell (D1) adjacent to said interlayer and composed of a semiconductormaterial having a fifth band gap smaller than the fourth band gap andbeing lattice mismatched with the third solar subcell (C1), andincluding a first contact on the top surface thereof, and a secondcontact on the bottom surface thereof, (b) a second semiconductor bodydisposed adjacent and parallel to the first semiconductor body andincluding: an upper first solar subcell (A2) composed of a semiconductormaterial having a first band gap, and including a top contact on the topsurface thereof, a second solar subcell (B2) adjacent to said firstsolar subcell (A2) and composed of a semiconductor material having asecond band gap smaller than the first band gap and being latticematched with the upper first solar subcell (A2); a third solar subcell(C2) adjacent to said second solar subcell (B2) and composed of asemiconductor material having a third band gap smaller than the secondband gap and being lattice matched with the second solar subcell (B2)and having a bottom contact; an interlayer adjacent to said third solarsubcell (C2), said interlayer having a fourth band gap greater than saidthird band gap; and a fourth solar subcell (D2) adjacent to saidinterlayer and composed of a semiconductor material having a fifth bandgap smaller than the fourth band gap and being lattice mismatched withthe third solar subcell (C2), and including a first contact on the topsurface thereof, and a second contact on the bottom surface thereofconnected to the terminal of a second polarity; (c) wherein the topcontact of the first semiconductor body is electrically coupled with thetop contact of the second semiconductor body and to a terminal of firstpolarity; wherein the first contact on the top surface of the fourthsolar subcell (D1) of the first semiconductor body is electricallycoupled with the bottom contact of the third solar subcell (C2) of thesecond semiconductor body; the second contact on the bottom surface ofthe fourth solar subcell (D1) of the first semiconductor body iselectrically coupled with the first contact on the top surface of thefourth solar subcell (D2) of the second semiconductor body thereof so asto form a five junction solar cell; and wherein the interlayer in eachof the first and second semiconductor bodies is compositionally gradedto substantially lattice match the third solar subcell (C1, C2) on oneside and the lower fourth solar subcell (D1, D2) on the other side, andis composed of any of the As, P, N, Sb based III-V compoundsemiconductors subject to the constraints of having the in-plane latticeparameter less than or equal to that of the third solar subcell (C1, C2)and greater than or equal to that of the lower fourth solar subcell (D1,D2).
 2. A multijunction solar cell as defined in claim 1, wherein theshort circuit density of each fourth subcell (D1, D2) being at leasttwice that of the solar subcells in each semiconductor body which areconnected in a series with the fourth subcell (D1, D2).
 3. Amultijunction solar cell as defined in claim 1, wherein the fourthsubcell (D1, D2) has a band gap of approximately 0.67 eV, the thirdsubcell (C1, C2) has a band gap in the range of approximately 1.41 eVand 1.31 eV, the second subcell (B1, B2) has a band gap in the range ofapproximately 1.65 to 1.8 eV and the upper first subcell (A1, A2) has aband gap in the range of 2.0 to 2.20 eV.
 4. A multijunction solar cellas defined in claim 1, wherein the upper first subcell (A1, A2) iscomposed of indium gallium aluminum phosphide; the second solar subcell(B1, B2) includes an emitter layer composed of indium gallium phosphideor aluminum gallium arsenide or indium aluminum gallium arsenide, and abase layer composed of aluminum gallium arsenide, indium galliumarsenide phosphide or indium aluminum gallium arsenide; the third solarsubcell (C1, C2) is composed of indium gallium arsenide; the fourthsubcell (D1, D2) is composed of germanium.
 5. A multijunction solar cellas defined in claim 1, wherein the interlayer is composed of p type(Al)In_(x)Ga_(1-x)As or In_(x)Ga_(1-x)P with 0<x<1, and (Al) designatesthat aluminum is an optional constituent.
 6. A multijunction solar cellas defined in claim 1, wherein the first and second semiconductor bodiesfurther comprise a first highly doped lateral conduction layer disposedadjacent to and above the fourth solar subcell (D1) and a blocking p-ndiode or insulating layer disposed adjacent to and above the firsthighly doped lateral conduction layer, and a second highly doped lateralconduction layer disposed adjacent to and above the blocking p-n diodeor insulating layer.
 7. A multijunction solar cell assembly as definedin claim 3, wherein the third subcell (C1, C2) has a band gap ofapproximately 1.37 eV, the second subcell (B1, B2) has a band gap ofapproximately 1.73 eV and the upper first subcell (A1, A2) has a bandgap of approximately 2.10 eV.
 8. A multijunction solar cell assembly asdefined in claim 1, wherein the band gap of the interlayer is in therange of 1.41 eV to 1.6 eV throughout its thickness.
 9. A multijunctionsolar cell assembly as defined in claim 1, further comprising: adistributed Bragg reflector layer adjacent to and between the third (C1,C2) and the fourth (D1, D2) solar subcells and arranged so that lightcan enter and pass through the third solar subcell (C1, C2) and at leasta portion of which can be reflected back into the third solar subcell(C1, C2) by the distributed Bragg reflector layer, and the distributedBragg reflector layer is composed of a plurality of alternating layersof lattice matched materials with discontinuities in their respectiveindices of refraction and the difference in refractive indices betweenalternating layers is maximized in order to minimize the number ofperiods required to achieve a given reflectivity, and the thickness andrefractive index of each period determines the stop band and itslimiting wavelength, and wherein the distributed Bragg reflector layerincludes a first distributed Bragg reflector layer composed of aplurality of p type Al_(x)Ga_(1-x)(In)As layers, and a seconddistributed Bragg reflector layer disposed over the first distributedBragg reflector layer and composed of a plurality of n type or p typeAl_(y)Ga_(1-y)(In)As layers, where 0<x<1, 0<y<1, and y is greater thanx, and (In) designates that indium is an optional constituent.
 10. Amultijunction solar cell assembly as defined in claim 2, wherein theshort circuit current density (J^(sc)) of the first (A1, A2), second(B1, B2) and third middle (C1, C2) subcells are approximately 11 mA/cm²,and the short circuit current density (J_(sc)) of the bottom subcell(D1, D2) is approximately 34 mA/cm².
 11. A multijunction solar cellassembly as defined in claim 1, wherein at least the base of at leastone of the first (A1, A2), second (B1, B2) or third (C1, C2) solarsubcells has a graded doping.
 12. A multijunction solar cell assembly asdefined in claim 6 comprising: a first opening in the firstsemiconductor body extending from a top surface of the semiconductorbody to the first lateral conduction layer; a second opening in thefirst semiconductor body extending from the top surface of thesemiconductor body to the second lateral conduction layer; and a thirdopening in the first semiconductor body extending from a surface of thefirst semiconductor body to the p-type semiconductor material of thebottom subcell (D1), a first metallic contact pad disposed on the firstlateral conduction layer of each of the first and second semiconductorbodies; a second metallic contact pad disposed on the second lateralconduction layer of the first semiconductor body; and an electricalinterconnect connecting the first and second contact pads, a thirdmetallic contact pad disposed on the second lateral conduction layer ofthe second semiconductor body; a fourth metallic contact pad disposed onthe p-type semiconductor material of the bottom subcell (D1) of thefirst semiconductor body; and an electrical interconnect connecting thethird and fourth contact pads.
 13. A multijunction solar cell assemblyhaving a terminal of a first polarity and a terminal of a secondpolarity comprising: (a) a first semiconductor body including: an upperfirst solar subcell (A1) composed of a semiconductor material having afirst band gap, and including a top contact on the top surface thereof;a second solar subcell (B1) adjacent to said first solar subcell (A1)and composed of a semiconductor material having a second band gapsmaller than the first band gap and being lattice matched with the upperfirst solar subcell (A1); a third solar subcell (C1) adjacent to saidsecond solar subcell (B1) and composed of a semiconductor materialhaving a third band gap smaller than the second band gap and beinglattice matched with the second solar subcell (B1); a fourth solarsubcell (D1) adjacent to said interlayer and composed of a semiconductormaterial having a fifth band gap smaller than the fourth band gap, andincluding a first contact on the top surface thereof, and a secondcontact on the bottom surface thereof; (b) a second semiconductor bodydisposed adjacent and parallel to the first semiconductor body andincluding: an upper first solar subcell (A2) composed of a semiconductormaterial having a first band gap, and including a top contact on the topsurface thereof; a second solar subcell (B2) adjacent to said firstsolar subcell (A2) and composed of a semiconductor material having asecond band gap smaller than the first band gap and being latticematched with the upper first solar subcell (A2); a third solar subcell(C2) adjacent to said second solar subcell (B2) and composed of asemiconductor material having a third band gap smaller than the secondband gap and being lattice matched with the second solar subcell (B2)and having a bottom contact; a fourth solar subcell (D2) adjacent tosaid interlayer and composed of a semiconductor material having a fifthband gap smaller than the fourth band gap, and including a first contacton the top surface thereof, and a second contact on the bottom surfacethereof connected to the terminal of a second polarity; (c) wherein thetop contact of the first semiconductor body is electrically coupled withthe top contact of the second semiconductor body and to a terminal offirst polarity; wherein the first contact on the top surface of thefourth solar subcell (D1) of the first semiconductor body iselectrically coupled with the bottom contact of the third solar subcell(C2) of the second semiconductor body; the second contact on the bottomsurface of the fourth solar subcell (D1) of the first semiconductor bodyis electrically coupled with the first contact on the top surface of thefourth solar subcell (D2) of the second semiconductor body thereof so asto form a five junction solar cell; wherein the first and secondsemiconductor bodies further comprise a first highly doped lateralconduction layer disposed adjacent to and above the fourth solar subcell(D1) and a blocking p-n diode or insulating layer disposed adjacent toand above the first highly doped lateral conduction layer, and a secondhighly doped lateral conduction layer disposed adjacent to and above theblocking p-n diode or insulating layer.
 14. A multijunction solar cellas defined in claim 1, wherein the fourth subcell (D1, D2) has a bandgap of approximately 0.67 eV, the third subcell (C1, C2) has a band gapin the range of approximately 1.41 eV and 1.31 eV, the second subcell(B1, B2) has a band gap in the range of approximately 1.65 to 1.8 eV andthe upper first subcell (A1, A2) has a band gap in the range of 2.0 to2.20 eV.
 15. A multijunction solar cell as defined in claim 13, whereinthe upper first subcell (A1, A2) is composed of indium gallium aluminumphosphide; the second solar subcell (B1, B2) includes an emitter layercomposed of indium gallium phosphide or aluminum gallium arsenide orindium aluminum gallium arsenide, and a base layer composed of aluminumgallium arsenide, indium gallium arsenide phosphide or indium aluminumgallium arsenide; the third solar subcell (C1, C2) is composed of indiumgallium arsenide; and the fourth subcell (D1, D2) is composed ofgermanium.
 16. A multijunction solar cell assembly as defined in claim13, further comprising: a distributed Bragg reflector layer adjacent toand between the third (C1, C2) and the fourth (D1, D2) solar subcellsare arranged so that light can enter and pass through the third solarsubcell (C1, C2) and at least a portion of which can be reflected backinto the third solar subcell (C1, C2) by the distributed Bragg reflectorlayer, and the distributed Bragg reflector layer is composed of aplurality of alternating layers of lattice matched materials withdiscontinuities in their respective indices of refraction and thedifference in refractive indices between alternating layers is maximizedin order to minimize the number of periods required to achieve a givenreflectivity, and the thickness and refractive index of each perioddetermines the stop band and its limiting wavelength, and wherein thedistributed Bragg reflector layer includes a first distributed Braggreflector layer disposed over the first distributed Bragg reflectorlayer and composed of a plurality of n type or p typeAl_(y)Ga_(1-y)(In)As layers, where 0<x<1, 0<y<1, and y is greater thanx, and (In) designates that indium is an optional constituent.
 17. Amultijunction solar cell assembly as defined in claim 13, wherein theshort circuit current density (J_(sc)) of the first (A1, A2), second(B1, B2) and third middle (C1, C2) subcells are approximately 11 mA/cm²,and the short circuit current density (J_(sc)) of the bottom subcell(D1, D2) is approximately 34 mA/cm².
 18. A multijunction solar cellassembly as defined in claim 13, wherein at least the base of at leastone of the first (A1, A2), second (B1, B2) or third (C1, C2) solarsubcells has a graded doping.
 19. A multijunction solar cell assembly asdefined in claim 13, comprising: a first opening in the firstsemiconductor body extending from a top surface of the semiconductorbody to the first lateral conduction layer; a second opening in thefirst semiconductor body extending from the top surface of thesemiconductor body to the second lateral conduction layer; a secondopening in the first semiconductor body extending from the top surfaceof the semiconductor body to the second lateral conduction layer; and athird opening in the first semiconductor body extending from a surfaceof the first semiconductor body to the p-type semiconductor material ofthe bottom subcell (D₁), a first metallic contact pad disposed on thefirst lateral conduction layer of each of the first and secondsemiconductor bodies; a second metallic contact pad disposed on thesecond lateral conduction layer of the first semiconductor body; and anelectrical interconnect connecting the first and second contact pads, athird metallic contact pad disposed on the second lateral conductionlayer of the second semiconductor body; a fourth metallic contact paddisposed on the p-type semiconductor material of the bottom subcell (D1)of the first semiconductor body; and an electrical interconnectconnecting the third and fourth contact pads.